There were two things that helped me. I think I needed to do both these things together, not just one or the other.

1. When I met my husband, I decided not to make him prove himself in ways he shouldn't have to. He should have to earn my trust. He shouldn't have to constantly prove he wasn't my ex-fiance.

2. I also made some firm dealbreakers in my own mind and actually wrote them down, kind of like a manifesto for myself. So, while I didn't expect my H to prove he wasn't my ex, I did have other expectations.

Before I came up with my list, I spent a lot of time studying other people's relationships and asking myself what I thought was good, bad, healthy, unhealthy, etc and worked out what to look for, not just what to avoid - it's much easier to look for good, functional, trustworthy behaviour than to look for the absence of dysfunctional or abusive behaviour, if that makes sense.