Welcome Saoirse. Sorry you find yourself here. All of us who have/had a cheater in our lives have been exactly where you are in that state ; broken-hearted and feeling complete bewilderment and abandonment. The atrocity you have experienced is completely inexplicable and incomprehensible.
The heart dominates, and the mind is dealt a shocking blow, wounded, and unable to function perfectly.
In this state it can be helpful to have a playbook. I'll try to give you some of that, but also read the archives here, there are many repeated patterns and your recognition of those over time will help you find your footing.
Before we get into that though, let's pick apart the actions of your cheater, under a well lighted microscope.
So, you lost a loved one (a death? severing a toxic relationship? what was it?), which is a cue in a plain vanilla marriage to LEAN IN and provide LOVE and SUPPORT. But in this backdrop your cheater developed a hidden resentment of the additional emotional labor you required. Presumably you were taking time for yourself to grieve and your spouse wasn't feeling appropriately "centered" as the top 1% human -- AKA "God's gift to YOU" -- that he was. So he then told himself he "wasn't happy" (they ALL then follow this with "for a long time" to emphasize their martyrdom, because somehow its harder to be them than us, because EVERYTHING is about them) and instead of talking with you about that, he kept it secret from you. He then justified to himself that because he was "unhappy" it would therefore be ok for him to commit a CRIME--the crime of adultery (which is NOT merely a marital atrocity, it is an blatant self destructive attack on EVERYONE around the guilty party [do you have kids? They have been DEEPLY harmed, they just may not know it yet. Is his accomplice married? Do THEY have kids? The spouse, the kids...ALL deeply wounded by this. The blast radius can be ENORMOUS]). I call it a crime and am pretty adamant about that because it is the deepest form of fraud. Under all circumstances it is a form of murder. It's a murder of the marriage and the family. It's a murder of the spouse (you)...in a sense the old spouse "dies" in this experience and becomes a different person. It's a murder of themselves (him). They are never the same to you. The person you knew and loved is "gone". It's a theft because "SOME THING" here has been stolen: a) secrecy and deception were used to b) obtain a benefit of some kind that was c) illegitimate -- otherwise it would have been done out in the open, no secrecy or deception required. From your side, he stole your time, your commitment, and your persistent presence in his life. If you have kids, were you home taking care of them while his was at the hotel with his accomplice? While he was plotting and scheming with her, he was holding you in place, testing his options and building an exit strategy that he would notify you of in a "surprise attack". If he had any courage he would have notified you of it and allowed you the same "luxury", but you might have beat him to the punch, and that would wound him further. He had to be the one leaving you, not the other way around. Is this a description of someone who is "marriage material"? Does this describe someone with a track record of behavior that seems like a "good bet" to be a partner and a protector? I can answer that for you. The answer to both questions is "NO". It's ABUSE. There's nothing loving or courageous about it. These behaviors show on not a single "what to look for in a spouse" list. In fact, they appear on every "how to disqualify a spousal candidate" list.
In the event your lost loved one was not the death of a beloved relative or friend but instead perhaps the loss of your own adultery accomplice (I hope that's not the case and merely say that as a disclaimer, I'm not attacking you with that comment) well then most of what I'll say here doesn't apply...but in the case your loss was exactly that (family, friend), then this saga I've mapped out (with PLENTY of assumptions) above is a pretty cowardly, selfish, criminal and unattractive set of behaviors, and you have to ask yourself "is this acceptable behavior --EVER, but most importantly THIS RECENT in a person's past (and presumably present...I would bet ACTUAL money he is still in contact with her and probably still intimate, its just gone further underground)-- in a person I will commit the REST of my life to? Thing is, you MUST ignore the sunk cost investment of 25 years thus far or your choices will be blurred by an irrelevant, distant past. You CAN'T get that time back but you can still build the future you want.
Okay, now for the playbook....
Separate, 180, No Contact/Gray Rock
Your spouse, being in a "fog" (which is merely "the choice to continue stupidity and self-centeredness"), is not safe for you. Unless he was communicating things like: "I can't believe what I've done."; "I'm so angry at myself."; "This is ALL my fault."; "I will do everything in my power to make this right"; and "What do YOU need to feel safe?", then there's nothing to work with or work ON with him.
DO NOT engage in marriage counseling right now. Most marriage counselors are by and large only equipped to deal with plain vanilla, "normal" stressors in marriage, good for "maintenance work" in marriage but they are not a good default resource for the post-adultery apocalypse. For you, individually, if you elect to seek counseling, do so with an individual counselor that specializes in trauma recovery. Otherwise, you may decide "there is nothing wrong with me, I really don't need 'help'. HE's the idiot, let HIM figure out if he wants to improve or stay dumb." In the end, as far as THIS goes, there really is no mystery to it worth exploring. He's simply CHOOSING dumb over smart, selfish over generous, criminal over virtuous. Does that mean YOU need help? Let me ask you: if he got a DUI, would YOU need therapy? If he assaulted someone at a bar, would YOU need therapy?
You might consider a very strong version of this 180 technique, which is called "no contact". If you have kids, then use "gray rock". This means, you cold turkey try to get the poisonous toxins of this person's selfishness out of your system. In no contact you literally don't speak to them, text them, email them, NOTHING. In gray rock you only discuss logistics involving the kids. You don't give advice, you don't carry their emotional baggage, you don't SERVE them in any way. You REMOVE the gift that is you from the equation, and you let them deal with your absence. MOST of the time in situations like this you're looking at someone on the normal spectrum (presumably you) and someone with some borderline or narcissistic tendencies (presumably him), where the tendency toward selfishness in the disordered person can either no longer be contained OR just finally comes to light (this may not be his first rodeo, but rather the first rodeo that risked exposure to you). Normal people, "feeling unhappy" don't go on a crime spree. They talk about it. F@cking someone else, when you have COMMITTED To MONOGAMY via a formal contract such as marriage, is NOT NORMAL.
Beyond the 180/No contact/Gray rock, you need to look at health, legal and financial issues.
You are unlikely to really know if there was or was not overlap in your physical relationship with your spouse and their interloper. You MUST get tested for STDs. Whatever you do DO NOT HAVE SEX WITH YOUR CHEATER UNTIL YOU HAVE THEM SIGN A POST-NUP AGREEMENT (and don't even consider reconciliation/post-nup agreements until you have some real confidence their commitment is real. That won't just magically happen with a foggy cheater and if it does appear, don't trust it). In states where adultery is grounds for divorce and possibly advantages you financially in divorce settlements usually you forfeit this option if you "condone" their adultery by resuming sexual activity with them. This is hard because of the emotionality of all this and some cheaters try to maintain control by drawing their betrayed spouse into that but... JUST DON'T DO IT.
Get a lawyer and review your options. Start the process toward divorce. You don't have to go through with it, and remember even if you do it isn't actually the end for you. You can always reconcile after divorce. But divorce is the great field leveling maneuver in these equations. It brings heavy consequences to bear on cheaters unrealistic expectations that they can commit such an atrocity, get away with it unscathed, and emerge later to "be friends" with their destroyed x-spouse.
Consider hiring a P.I. in the event you need more information/proof, and consult your attorney about timing of collection of this information and your separation.
*Pro Tip: Sue the Other Woman
Consider suing the other woman. If you live in an "Alienation of Affection" state you can do this. In North Carolina six weeks ago a $750,000 judgement was granted a plaintiff who sued his wife's adultery accomplice. Judgments have gone as high as $9 million in that state. She has harmed you way worse than a fender bender and the hitting party in a fender bender gets sued MOST OF THE TIME. Don't listen to gaslighting arguments that "oh, HE is the one REALLY to blame, why go after her?" that's an unecessary misdirection. Of COURSE he harmed you, which is why he will face consequences TOO. But she interloped on your most precious, irreplaceable, and priceless asset: your family. Don't take that lying down. Have a court acknowledge the harm you've endured and collect the damages that are rightfully yours.
You would be wise to separate financially. Consider auditing all your joint financial affairs, calculate your net worth and take half and move it into your control. Brokerage accounts, checking accounts, retirement accounts, etc. Include the equity in your home. Don't forget collectibles and precious metals. Run your credit. Run his credit. Make sure you account for ALL the debt. One trick that is available to your cheater but hopefully he's not wise to is using small business credit to hide debt. Those accounts don't report to normal credit reports so they can be used nefariously. Reflect on whether he has that kind of savvy and figure out a way to probe... Anyway, get that all organized and under your control. You don't want to find yourself in a situation where he is using reconciliation and fog to drain your accounts or take on massive debt. You already "don't know" him, and you cannot trust he will treat you ethically at all...in fact he has shown you he won't, so TRUST THAT. Don't overstep, only take half, and you can justify that in any court (I am not a lawyer just a lay person so check with your attorney).
Contact the other side
If she is married, consider contacting her husband ASAP. This will remove ALL the power from them and place it where it belongs... in your hands and in the hands of her husband. The two of you are getting screwed over the worst. Your husband does not deserve your protection from the consequence of what may come from his actions, and neither does his accomplice. If they didn't want that to happen, they SHOULDN'T HAVE DONE THIS. Make sure the other party knows, and use your access to them to gain and offer useful information. See if your timelines match up. See if they intercepted any communication that is useful. That transparency can give you a glimpse into what REALLY happened, and it may be just what you need to get clarity. DON'T be vulnerable to gaslighting accusations that this is somehow disloyal or "an EQUAL betrayal" in comparison to what they did. No, they committed a crime. You are investigating. Of course, do this safely. Invite the person to meet in a public place, with people around, but preferably out of your zip code. You might have a trusted male friend/relative seated nearby "reading the paper", in case of anything...
One of the most difficult things to do in the aftermath of this kind of devastation is to seize your power and take control of your life. The temptation to let your emotional state take over your life will make you more vulnerable to attack and further deception. I wish I had someone way further down the path telling me what to do when this happened to me. I languished in agony for a long time. It wasn't until I started putting a real strategy in place, a real investigation into the whole thing, and got ACTUAL information: intercepted communications, personal info about the other party, etc -- stuff therapists tell you NOT to do/NOT to look at -- that I got clarity, and was able to start moving toward feeling ANYTHING like normal again. Read anthro's INCREDIBLE account in one of the recent threads describing the feeling of never being the same again, YEARS after the fact, that resonates with SO MANY of us here...
You Didn't Cause This. You Don't Deserve This.
Know this Saoirse (the one assumption being you didn't cheat ahead of him), nothing you did prompted this. He is a grown up, not an infant, and he is in FULL control of his choices. He chose to bring this calamity down on you Ups and downs in life are NORMAL. Grieving the loss of loved ones is NORMAL. And going through that with our partner and offering love and support and comfort is what marriage is about. If we turn our backs on our partner when they are suffering, and go on crime sprees we don't get to come back to them claiming their ups and downs had anything to do with our crime spree. We have to own our stupid crime sprees, and go to jail ourselves....we don't get to take our spouses with us in there. And there is no "unhappiness defense" to crimes.
The tendency to long for what we've "lost" (or for what we thought we had) is a natural knee jerk reaction to this kind of betrayal and abandonment. In my opinion it does a disservice to betrayed spouses and leaves them vulnerable to manipulation. I think the right move is to plan on leaving and organize to do so. Leave it to the cheater to seek you out and make a play for you if they have the courage, but otherwise put walls up, and defend your heart. If they come knocking put ENORMOUS proof requirements on them. Like "Oh, NOW you want me? Ok, you're going to tell me everything, and THEN we'll have a polygraph verify it. Sign this post-nup that puts the risk of success of reconciliation on YOU. The post-nup bans burner phones and other circumvention techniques and you agree to GPS tracking, whether I feel like monitoring it or not." Make them EARN another chance. How can they appreciate it otherwise? But even then, the only way forward is some kind of probation, and who wants that? Bleh!
So sorry you are here. Hope some of that is somehow helpful to you.