Thank you for so many replies. I appreciate them, even if some are hard to read. I'll try and answer some of the questions.
After finding out, I did the pick me dance and realise now that I was hysterical bonding. I feel sad that I was so desperate to win him. I have told him I wish he had actually left when it started so he didn't get to treat me the way he did. And for me to allow it. He blamed me for the affair, well for him wanting space then the affair.
We have seen a relate counsellor together as well as individually. She was very good but I think we both wanted to move forward at the time. We both ended up having the OW as a common enemy as she had started her stalking at this point.
We do have a physical relationship and have done the whole way through this. It has dwindled to once a week now and it has always been good.
The affair, I now realise after beating myself up, was all about him and his MH issues. At work he could be someone different and so was she. Neither of them were real with each other. He wanted something he could control, that his MH wasn't affecting. Its a very complicated condition.
We are already late 40s and I'm not even thinking that I want anyone else! If that happened later down the line that's fine but not being able to get another partner is not a reason to stay for me.
Some of you are right, maybe I reframe this as, I will see how things are in 5 years rather than planning to leave.
I know that things can change and I am open to either option. I don't hate him for what he did, it just makes me sad. I also realise that someone can make bad choices but it doesn't make them a bad person. I absolutely believe he will never do it again.
A big part of our issues is that i would like to discuss my feelings and thoughts about it but he won't. He can't cope with the realisation of what he has done. He loves me more than he ever has done. In his mind this has been a second chance and our future is beautiful. I'm just not on the same page!
One of the things he said to me, 4 years ago, before I knew anything was the 'I love you, but I don't think I'm in love with you. I couldn't believe it, it was like an out of body experience and I was looking at two strangers. But now I wonder if that's how I feel.
@celticmissey please feel free to direct message me if you wish to share your story. I'm sorry you are there too.
Again, thanks for the responses.