"and I managed to move on" - the fact is, you haven't moved on. If you're still thinking about it, looking her up, hurting, still don't feel good enough and worry about him leaving you, then you haven't moved on. You've moved from one issue to another - instead of thinking about the cheating, you've internalised it and now feel inadequate and unsafe.

"We've never really talked about what happened" - Should have, and that conversation should have been the deciding factor on whether you got back together. "aside from him admitting he treated me horrendously" ? Did he say why and work out a plan for how he was going to fix himself, and also find a way to guarantee you it would never happen again?

What happened wasn't your fault. It wasn't your fault you were cheated on. Even if you were the worst girlfriend ever, he could have done any number of appropriate actions (umm...talked to you?), but he chose to cheat and then the issue was never resolved. I'd like to make clear that it is not your responsibility to resolve it or to seek these things out from him. If he was truely sorry and regretful about what he'd done, he'd probably be a mess about it and actively seek to do the steps I listed above - 1. Recognise fault, apologise and explain. 2. Fix the original issue. 3. Make amends with partner and make a solid commitment/guarantee that it will not happen again. If he hasn't, then these things need to be done. See a marriage counsellor.

Sorry to sound blunt OP, I'm not attacking you, but this is obviously affecting you to the point it's seriously damaging your self esteem and it needs to be fixed for you to be happy in your life and yourself.