OP I've been going though this for the past 18 months. With DH it was a brief emotional affair (she thought he was fucking amazing, so did I but I didn't tell him that everyday because we both had jobs, kids and responsibilities) but the disassociation was there. It was like the man he is shut off entirely and was replaced with someone who was a sort of vacuum. Even in photos of him taken when it was going on he looks different.
In the beginning it was incredibly hard and a lot of my friends knew because I broke down a couple of days later and we were at a group activity. None of them told me to leave him. They all said how completely out of character it was and have all been supportive of both of us.
A few said that you have to truly forgive to get anywhere or you'll drive a wedge between the pair of you that can't be removed. That's the hardest thing. Knowing he made choices which hurt me, as I was asking him WTF he was doing for weeks before I found out. He saw the pain I was going through but chose to lie as to keep his little relationship with her going. A ONS would have been easier in some respects I think but then it wasn't and I've not been there so I can't say for sure.
He had the sexual health checks, went to counselling alone. We went together as well and he tried everyday. He's still trying. It's hard when you look at them and they look like the same person but to you they aren't. They've made this choice that you had no part of which has changed your life forever.
They'll be a lot of LTB on here, I had it but they are still the person who you fell in love with, who weathered the shit bits with you and who you grew with.
If you think you can truly forgive him and he is remorseful you have a good chance, you're only young and if it doesn't work out you won't be at the stage where you can't move on. He admitted it all which I think says a lot about him.
We've both reacted to things in different ways, I think he's never had open affection in his life (other than from me), his parents are very Victorian and I've never heard them say they love him or are proud of him. Then along she comes and he can literally walk on water, he's clever, handsome, funny, successful and wanted. I had a dreadful upbringing and tried everything to make life perfect which for me and in a way him a huge mantle to carry. How you grew up and what you've been through makes a huge difference to how you react in situations. Especially his relationship with his mother, it all sounds pretty horrific and although is not an excuse it sounds as though it does affect him greatly.
It's really really hard but not impossible. There will be shit days where you feel like your chest is compressed and you want to reach out to him to be comforted but have this weird conundrum that he was the one that caused the pain you need to be comforted for. Do you grab on or push him away.
Take time, really talk, think about what you want going forward, is this the man you want to make your future with, be the father of your children. Don't make rash decisions.
You're not any less than those who say cheating would be their line in the sand, it's a harder path to tread to not LTB but it has to be right and you have to get to the point where it's not the first thing on your mind when you wake up and all your decisions aren't clouded by this one thing,
Sorry, a lot of rambling there.