I feel for you liga. It is hard to not obsess about past transgressions and how they have affected you. With OCD quite a bit of the time things that other people can shrug off stick with us and shape the way our disorder effects us.
For example: My ex was a cheater, a liar, a user, and she constantly made me feel like shit about myself. There are so many things that I want to say to her, but I know it falls on deaf ears. I can only accept that those things happened and move on from them. Unfortunately for me I am driven by precedents and silly rules. Those thoughts and feelings I have towards her are slowly seeping into my current relationship...and yes something happened that reminded me of how things were, but I need to give my SO the benefit of the doubt. I will never know what exactly happened and I can obsess and ask questions all I want, but it only hurts me and makes her feel sheepish. All I can do is come to a place that I can accept what happened and that I will have to be okay with moving forward without knowing if I want to stay with her.
My lack of trust in people comes from how I was raised. The people you are supposed to be able to rely on were pretty shitty to me. I became angry and I learned that it is better to protect my heart rather than express how I feel. The things that happened growing up caused me to always expect the worst to happen in any situation. I guess I don't trust because growing up I could only really trust myself and my brother.