I am sorry you are hurting. Really. I may be a former BS (betrayed spouse) but I find no solace in the fact that APs (affair partners) get hurt. Pain only begets pain.
The end of any relationship is hard - the end of one that was based more in longing and fantasy vs. everyday realities, even more so. Nothing can, or will, measure up except perhaps another relationship built on longing and fantasy. Real life always takes some of the sparkle off.
All the above points are valid - I'd like to mention a few others that came to mind.
Right now you see him in this gallant, tortured hero role. Someone who is sacrificing his (and your) happiness to be a good father. Right?
I would say wrong. If he'd chosen to work on his relationship with his wife before starting an affair - then he'd get to play the long-suffering hero role. However, here's what he is really doing:
He's maximizing his comfort and happiness - while stealing his GF/wife's and yours.
He's unwilling to lose time with his kids - so he stays with her, likely telling her he's crazy about her - while not being truthful that the parameters of their relationship have changed. She's still playing by the old rules and trust me - he likes it that way. He doesn't want to have HER having sex with someone else and getting all romantic about them That'd be a knock to his ego. That's the mother of his children! He hasn't told her he wants an open relationship so she could be free to leave or have her own fantasy fun on the side. Maybe it really is only about his kids or maybe it's because he wants her and
you. Regardless it says he's willing to steal her opportunity to be loved the way she deserves (or chose to live in an open relationship) because HE doesn't want to experience discomfort. That isn't heroism, it's selfish.
You are being equally selfish with your boyfriend. Do you think he deserves to be lied to? To stay with someone who will likely cheat again as she feels no "passion" for him? Who seems to see him as less of a catch than a guy who cheats on the mother of his 3 kids? I think not. He's clearly a catch in his own right, as you've explained. Tell him the truth (all of it - including the lack of desire). It may save him a lifetime of wondering why he feels like he never quite measures up. Giving him half of your love is not kind, it is selfish and cruel.
Here's one more terrible tidbit to consider. The MM is one of two things - a serial cheater or a one-off affair. If he's a a serial cheater - he's such a consummate liar that you won't likely know it and believe everything he told you hook line and sinker.
If this is his first and only affair, his feelings for you (and yours for him) may, in fact, be far more about where you are in life - then each other. I am not saying it isn't possible that you are star-crossed lovers that somehow never found one another - it's possible. And I know that he's telling you that he's never felt this way before. But he's lying to her right now - what makes you think that he's not lying to you (or even himself) as well?
Here's a scenario that seems to play out all too often with the one-off affairs (male and female):
In the beginning, he and his SO were very in love - but three kids changed the dynamic in the relationship. She started putting all the time and energy she used to pour into him into their kids. He began to feel taken for granted, pushed aside. He wanted that great feeling where he's the center of attention and grew resentful. Somewhere in here - you walk in the door. You admire him, laugh at all his jokes, think he walks on water. It feels amazing. It hasn't been this way with her in years. He feels like he loves you, adores you. It's a dream!
But then DD comes and he has to face losing you or her. He starts realizing that he doesn't want to visit his grandkids with you some day. He doesn't want to see his GF/wife with another man. And as the consequences of losing her (and their family unit) become clearer to him - the fantasy with you becomes far less appealing. He starts to realize that what he was in love with was how you made him FEEL. And now she's refocused on their relationship and him, he sees that they have a chance to get back to where they used to be and he jumps at it. Because THAT is what he was really looking for - not a new mate - but a chance to recapture that feeling, of being special, wanted, needed, desired.
But how can he tell you that? How can he say that he has realized that he really does love her and that he made a mistake getting involved with you? That he should have tried to rekindle the fire with her for his own sake, her sake and their kids sake? That in his weakness and selfishness, he used you. Remember - this is the same coward who couldn't tell his wife he was feeling ignored the first time - so he doesn't tell the truth to YOU either. He says that it is just for the kids, that it is really you he loves. Because that keeps him looking good in your eyes (and he DOES care about you.) Of course, once again being selfish and leaving you thinking you just lost the love of your life, when in reality, that "love of your life" is working hard to save his relationship with his wife/GF.
About 4 months after no contact with his AP, my husband and I were talking and he said that one of the things that he was most ashamed of was how he'd convinced himself (and her) that the relationship was something more than it really was. That at the time it had felt like he was deeply infatuated, maybe even falling "in love", but after DD it was like the world changed on its axis. Suddenly, he realized it was all about HIM. He was in love with the reflection of HIMSELF she gave back to him and all the attention. He couldn't believe that he had hurt me, her, our daughter and himself for a feeling that practically disappeared overnight. He still felt guilt and shame for hurting her - but not one ounce of regret over his decision.
Unfortunately, it took him making that HUGE mistake to make him realize that he had been being a coward about asking for what he wanted in our relationship and then in his resentment that I wasn't meeting his "needs" - acted out to get those needs met elsewhere. He got serious about becoming more authentic and willing to own his needs - and in doing so, became more "integrated" (having integrity). This allowed him to be someone he genuinely liked again (he HATED who he'd become - he despised liars and couldn't believe he'd sunk so low.)
I see often on this forum that for the one time cheaters - this is not a unique story. They reach an unhappy point in their life, feel invisible, resentful, depressed, etc. - and start an affair thinking it will cure all their ills. Then DD comes and they realize they've only made their own problems worse - and drug someone else into it. And IF THEY ARE LUCKY, their spouse is willing to stand by their side while they FINALLY get to the bottom of the real problem, THEMSELVES.
This might not be your MM - but there is a very good chance that it is. It is RARELY just about the kids. Most people do what they WANT to do. He obviously had no problem doing what he wanted before. I knew a couple once that fell out of love but both wanted to be with their kids full time. So they shared a house that had a Mother in Law suite that the husband moved into (separate entrance, etc). They both had BF/GFs, while they raised their kids together. With honesty a lot of things are possible. Your MM doesn't either know how to deal with difficult situations and work towards compromise - or he doesn't WANT to leave her. Neither make him attractive for you.
You have taken yourself down a very dark rabbit hole. You have been party to lies to your own SO - and party to the lies the MM told his wife - both before DD and after. You stole an opportunity from her to know the truth. But now sadly, EasyasABC is right - you can't go back and undo it. The likelihood is she will do some digital digging - and she may uncover his dishonesty on her own. If she contacts you again - be HONEST. And be ready to hear that you may have been told some BIG untruths about the state of their relationship as well. But do not reach out. If she wants your input, she will come looking. Otherwise, focus on YOU and how to get back to a healthy, honest place in YOUR life.
Because you may be a victim of his lies, but you are leaving victims in your wake as well. Unfortunately there is always a price for selfishness and deceit. Maybe now, maybe later. But always.
Start living authentically. Forget about him. Save yourself and remember - if you can't be honest about it, it isn't worth doing. REALLY.
You are human, you have made some pretty big and ugly mistakes. You have not shown great courage or character in this situation. You stand at a crossroads now. You can be the person you have been being - or start being someone stronger, kinder, more honest with more integrity.
You have been putting your emotions first and your values second, That is what got you here. Try putting your VALUES first and emotions second. I think you will find that in 6 months to a year, you will be in far happier and more integrated place.
Because that is the great thing about being human - we may be fallible - but we are also always redeemable. You just have to own your mistakes - and go to great lengths to learn from them and not repeat them.
Wishing you peace and clear thinking during the months ahead.