I too had exactly this situation. DP and I had a very good relationship, we talked, openly, we shared thoughts and dreams, we told each other how much we felt and our physical relationship was excellent and very well matched.
5 months ago I sensed something was 'off', there was nothing I could actually put my finger on, but there were odd days when he definitely went colder towards me. He went back home for a school reunion (of all things) and contact for those 4 days was not the same as previously. I even asked him, while he was there, whether there was anything I should be worrying about, he reassured me...
2 weeks after he returned, I asked him something fairly simply, he paused, I pushed, then he confessed that he'd been having an emotional affair. I kicked him out. He wasn't expecting it because he (like many men his age - 50s) assumed no sex made it easier. It doesn't. the betrayal still cuts right through you.
I went through hell, and back. He begged to try again so we did. We go the book, we worked through it, we talked, long and into the night, we cried, we had amazing sex.
Then, 4 weeks ago I started to get 'that feeling' again. I couldn't work out whether I was just being paranoid, I doubted myself, I asked him.....
We split up 2 weeks ago. I am beyond devastated. I have no idea whether he is with the OW or not. She's married, with 2 children (teenagers). I've no idea whether she had to sit those kids down and tell them that she was leaving for another man.
I spent last weekend clearing his stuff from the house, it's in a large pile until I can summon up the courage to ask him to pick it up.
So, this is long, but you need to make your decision with full information, and this is the view from the other side. For me, the worst part of this whole process was the time when he was having that EA and assuming I didn't know. So your OH does know, he may not want to admit it to himself, because he trusts you and loves you, but every time you choose to contact the OM, you are picking up that trust and love and stamping it on the ground. It's always a choice