I can only make my best guess from the outside but I think where your husband is at is simply a place of very great pain that he has basically no relief from. And what he may be saying you don't get is that there is nothing you can say or do that can make that pain go away. In his case the whole last 20 years feel false. He can't think back on any memory and feel like it was real. Literally everything is contaminated. It is very, very crazy-making. It is not strange to me that he is in a permanently unbearable state. 

But.

He also needs to get something, and he doesn't seem to be getting it. It is that you both havework to do. Working out how to live - and like, live a life, not just barely survive - with the pain that you cannot relieve for him is his job. It is his obligation because he is the person living his life. He got one life, he may not like it right now, but it's his only life and we all have a duty to get into it and make it as good as it can be. We have a duty to ourselves and the whole world of people around us to have a worthwhile life and contribute to the world, and not to let pain cripple us forever. 

He keeps looking to you to solve his pain, but there is no fix outside himself for it. It may be your fault but that doesn't mean you have the solution. It's his job to make his life worthwhile and his journey to find a way to come to terms with all the stuff that's tormenting him. You may have dumped him in the middle of a seemingly endless forest, but only he can find a way to walk out of that forest. He has to listen for the river, find it, have a drink, then follow it back to civilisation.

You meanwhile are in a different forest with a different river to follow.

He seems to be doing the same thing over and over and so of course he is not making any headway. He needs to try some new approaches and see what actually makes his life better day to day. 

I'm not completely certain how this fits into your question or if it addresses it some way, but what jumps out to me is that you've had nearly 20 years to digest the entire affair dynamic and he's not reached his second year D-Day anniversary. Hence, your ability to see what he's seeing, feel what he's feeling, etc., is hindered by the differences in time.

What's real to you is, you had an affair 20 years ago and have come to terms with what it means. You've read, studied, journal-ed, been counseled, and have had time to reconcile its place in your life and how to deal with it. What's real to him is the last 20 years, (or 50-75% of your marriage??), isn't what he thought or lived. He was someone else because you were someone else. Two decades of questions without answers. That's a long time of different perspectives for a married couple.

Perhaps you haven't thought of it this way, but your husband is thinking back to 20 Valentines' Days and 20 Wedding Anniversaries that are now in question. Imagine you finding out at 21, you were adopted. Your mom isn't your real mom. Your dad isn't your real dad. Your siblings aren't your real siblings. You're not who you think you are and since you aren't, you begin asking, who am I?  Those are thoughts and questions that require answers, or fill an emptiness that was previously not there for 21 years and now all of a sudden is there.

Which segues into my next point...... the lack of facts and details that would have lead him to answers or hypothetical conclusions. Your inability to provide  facts and details, hinders his ability to reconcile the last two decades. Although now forgotten, once upon a time, you had these details and having them enabled you to move forward with the rest of your life in a manageable way.  His restarting point was a mere 15 months ago. In a 100 meter race, you're at 80 meters and he's just approaching 10 meters.

The advantage is an uneven one. Using the adopted child analogy, the adopted person who can find their biological parents can start to reconcile their life and move on.Thoughts or questions concerning physiological differences between them and other family members are no longer (back in the mind) questions marks. Now, he or she knows or can at least speculate why they're shorter or taller, lighter or darker complected, more or less athletic or artistic. Relief to some extent is achieved, along with a stronger self identity. 

The ones who can't find their biological parents live with this loss of self identity for the remainder of their lives.Their siblings, friends, classmates, workmates, etc., can't relate to this, much like your inability to your husband's place in life partly because he is just finding out who he was married to a mere 15 months ago.

Another analogy might be him awakening from a coma that lasted years and unable to fill in the activity that happened while in the coma. There's permanent loss of a period in which he knows some very significant things occurred. The kids are half a foot taller, the wife is older and grayer, etc. Considering you can't remember anything, the most meaningful and life altering period of his life is lost.

Not only were you not the person he thought you were, but he can't put pieces together that would help him establish  a foundation for reconciling everything that has taken place. He has no such basis and has no chance of redeeming it without information. 

These are just thoughts, not necessarily truths. Just some things for you to think about. I'm sure much more relevant information is involved that I don't know about, so take what's relative and throw out what's not.

I absolutely agree with everything already written, but wanted to add, the level of pain a BS feels is absolutely indescribable.  No matter how much he explains his hurt, and you listen, he will never be able to explain the depths to which those feelings go, the magnitude those feeling have.  You may try, and that's admirable, but you will never truly "get it".    When he try's, and he realizes there is just no way to describe it in a way you can understand, it will leave him frustrated because he knows that's at least one hopeless part of this whole situation. 

As Blessedby7 indicates, one simple word may provide you with answers to your questions. Pain. I didn't mention it earlier because it's a given, but perhaps it requires more attention. The pain that has led you to give thoughts of suicide, is fractional of the pain he's feeling. I was betrayed twice by 2 fiance's and they are more painful than what I experienced from my dad's death. Both fiances occupied approximately 4 years of my life, however my dad fathered me 70 years.

Logistically this makes no sense, but logic doesn't occupy any part of the definition of betrayal pain, which arguably, has no peer notwithstanding the death of one's child perhaps. Who knows? The difference is incomprehensible. Your husband's pain level may exceed your understanding of it by a long shot. Plus, it's hard to understand someone else's pain when you are wrapped up in your own. 

When I go to my chiropractor, I fill out a chart that asks what my pain level is on a scale of 1-10 with 10 being the highest. If such a question were asked by therapists, many BS would ask write in a higher number than the 1-10 range, believing 10 is inadequate in defining betrayal pain, with respect to depth and longevity. 

It's consistently stated on infidelity forums that it takes 2-5 years for BS to recover. I can't substantiate this at all, but if true, he's still 9 months shy of the MINIMUM time period of this estimation, so commit to anticipating and preparing for his pain expressions. BTW, you can shave that 15 months down by the same amount of time you were threatening to divorce and berating him to get over it.

At that time, he was not on the path to recovery, he was simply trying to survive, one day at a time, literally and figuratively. No healing was taking place, so you can deduct that period length from the 15 months and you have an accurate length of time  where he is in recovery with your support. In other words, the months of which you were remorseful and supportive, are the months that can be counted towards his recovery. 

Edit Note: After rereading your post, I want to add that being married 23 years and finding out the affair took place 19 years, coupled with your inability to recall a significant amount of it, your husband is questioning nearly the ENTIRETY of his marriage, which may account for what? ........ nearly half his life, give or take a few years? His ability to decide on his future was not afforded to him because he was left in the dark. His life was being altered and he had no knowledge of it happening. He shared his wife with another, but was clueless. 

Furthermore, he's trying to reconcile, but unsure of what or who he's reconciling with, as aspects of the affair are unknown to him and you are still unknown to him. It's not much different than signing a contract without knowing the terms and conditions. As you can imagine, significant anxiety can be felt signing a contract without knowledge of what the person is agreeing to.

I'm sure this hurts, but I hope it helps. I commend you for revisiting this site, per your husband's wishes and I'm wondering if it wouldn't be a bad idea to share with him some of the responses here, so that he can get reassurance that others can relate and attest to his situation. And again, apply what's relative and dismiss what isn't. Well wishes to you both.

For the bast 3 weeks he has been asking me for a divorce and I keep begging him not to leave me, but I think he has finally made up his mind.

We have overcome many obstacles. We have had wonderful memories. We have started over financially a couple of times in our marriage. We have been able to pull through extremely hard situations, but he says he is tired now and can no longer fight for what was a lie all these years. I can’t accept this. It’s unbearable to hear him say this after so much we have lived together.

You might be at risk of creating a shared pattern where he says this and you beg and that keeps repeating, because a part of him will be getting some relief and sense of regained power from your begging. That's not acceptable if that is what he is doing but it is somewhat understandable given that he feels so disempowered. 

It would not be a healthy pattern. These things can be very subtle and complex and only you can read your situation but it may be that you have to say, "Divorce is not what I want, but if you're sure that's what you want, then okay. Let's get the ball rolling so you can get on with your life." 
 
He may need to really see separation as real, otherwise he can keep seeing it as a way to assert. power over you and get the gratification that comes fr you begging.

I am sure he doesn't see it as having been done together. I don't perceive anything my wife did while deceiving me as authentic. I'm sorry but you were living a lie, you weren't really there at all. Every sacrifice he made while you were being untruthful seems to him to have been scammed out of him, like a big long con. He will be thinking about how he wishes he had put his effort into something real rather than something false. 

Sorry but that's how I see it and he probably does too.

It sounds as though he hates what he has become as a result of all this. I hate what I am now, with great intensity, this broken half-there thing I am. It's hard for him to find a path forward that isn't repulsive to him.

You probably have to let him take these steps wherever they may lead him. It is not a certainty he'll leave but I think you have to let him go so it's all his free choice.

I'm sorry.

He has mentioned how in reality we were only married for a couple of years if we really analyze the relationship. This is extremely hard for me to hear. We have lived many things together. We have overcome many obstacles. We have had wonderful memories. We have started over financially a couple of times in our marriage. We have been able to pull through extremely hard situations, but he says he is tired now and can no longer fight for what was a lie all these years. I can’t accept this. It’s unbearable to hear him say this after so much we have lived together. I know I have to come to terms with it because it’s not about me, I have to think of what I would do to see him happy.