I really sympathise. If there's a level of confusion about his behaviour, about what happened, then your mind will want to fill in those blanks, but it can't.
That's what it was like for me, and I will never really understand, because I can never really trust that I'll have the truth from him. I realised that I had to trust on my own instincts, because I couldn't rely on his words any more as they were in contradiction with his behaviour. However, I'm not telepathic, so even though I think my instincts and objective opinions of him were correct, it's not a fact in the same way that gravity is a proven fact, you know? You don't have the same sense of closure, and if you've been with someone for a very long time or were heavily invested in someone, getting that strong sense of closure and understanding of not only someone's actions but their personality is more important.
If you went out with a guy who was known to cheat, for example, then I don't think you would find him cheating all that surprising, because it's what you would have understood about him in some way from the beginning. If it's surprising, then you have to deal with the fact that either he genuinely changed for the worse over time, or that he his that part of himself from the beginning, or you just him wrong. Understanding it as a type of grief provides a different perspective, I find. It's a powerful loss, even if they're not dead, and even if on surface level they say they have regrets.
Just remember that highly unpleasant people are capable of regret and great distress, but it's rarely true, long lasting remorse or regret. It's because the power dynamic has changed, now that you are at a distance, or because of some kind of panic that they've lost some security, because they now have to stew in their own juices and they don't actually like themselves, or because they are annoyed because they don't have what they used to have in other ways.
I wouldn't encourage you to move on prematurely, but sometimes what your mind needs to stop ruminating over past times is a new man in your life!