<human>: I'm F(22) he's m(25) 

Backstory: We dated five or so years ago for five months, then he broke up with me for his most recent ex. I never found out the details, but the woman's ex said that he was cheating. I'm not sure. I just assume he didn't, since he stuck to saying he didn't. They dated for four years, then broke up about a year ago. He still talks to her, has pictures of her on his facebook and talks about his 'past' with her. All of this makes me completely uncomfortable, and all I can ever think about is him getting back together with her. We've been fwb for a few months and I know we could possibly turn into something more. (He thinks of me as a girlfriend) 

Here's my problem: The past messed with my head a lot. He was the first person I had sex with after a Rape, and my first real relationship. He broke up with me for this other girl, which he basically wanted to marry up until about a year ago. He said he was miserable for the last few years, but didn't realize it until they broke up. Everyday I wake up expecting him to get back together with her. I don't want a repeat of four years ago. I can't shake these trust issues and I feel like they would affect any relationship I would have with him a lot, if we were to become something bigger. Does anyone else have this problem? Any advice? I'm at a standstill. Sometimes I just emotionally detach myself from him just because of it. He notices, and I feel like shit for doing it to him.

Can you give me the gist of the text in a nutshell?
<bot>: We broke up five years ago because he wanted someone else, they broke up after four years and we are fwb, I don't know if I can trust him for a relationship.