Glad you're able to get through your pain enough to at least be open-minded. I TOTALLY get the whole "being the doormat because I don't hold a grudge" thing (not that you said you were a doormat, but sometimes being very forgiving can feel that way) and that being the case, I would absolutely get to a therapist FAST so they can help you walk through your feelings and help you advocate for yourself. It's clear from your first post that you love him. It will be all the harder to stand up for yourself if need be.
On one hand, I totally agree with 2xy. For me, it's less about some people handling monogamy and more thinking that people that cheat have esteem issues that (unresolved) will always result in needing to feel "wanted" or "worth going after" even though they're married kinda thing. I've known two people like this--one that cheated, and on that only went after married people (as a means of feeling like she was "worth the risk"). :/
But I also think there really are other possible, valid explanations. Especially in light of the fact that he takes Ambien. My SIL has had to videotape my BIL because some of the things he said during Ambien episodes were getting terrifying. He rarely did anything physically and nothing he did resulted in danger, but the experience is enough that I could totally see that playing into it.
Do keep us posted. We're here.