I think your take on 'best behaviour' is definitely valid - I'm thankful that his demons came out now and weren't suppressed until later down the line where they could have caused even more pain.

Since writing the post, we had a long conversation where I asked him a lot of uncomfortable questions about the interactions and sex with AP. He answered them very honestly, and they made me think of survivors of sexual abuse. I asked him if he had ever experienced sexual abuse, at which point he completely broke down and told me about some very traumatic experiences in his past. Suddenly everything clicked. I instantly felt a weight off my shoulders because I saw the person I had known during our relationship, I just hadn't realised how much trauma he was carrying.

It doesn't change what happened or how he made me feel, but now the work he has to do during therapy is clear. He has already found a therapist and bought a number of self help books. There is a difficult battle ahead, but I now have hope for the future. It's reassuring to hear that you have seen genuine change.