We've been dating for almost a year now and I know that objectively he hasn't done anything that should lead me to believe this and it's just BPD brain not being able to move on. 

But a few months before we started dating he hooked up with this girl twice (he's always insisted that it was completely platonic and he had no feelings). Once he started liking me though, they stopped hooking up and maybe only hung out casually a few times more. When we finally did start dating officially, on the very first day we hung out, he told me that he was going to hang out with her later that day and asked if I was okay with that. He was really upfront and direct about it, and said that she had been asking him to catch up and hang out for a while and he felt weird just cutting her out like that with no explanation, so he was going to tell her that he couldn't continue the friendship. All this sounded solid so I didn't make too much of a big deal about it. But then for a couple of months I would notice that occasionally she would randomly call him or text him and he would respond. He wouldn't ever hide it from me and he would even tell me that he thought it was weird that she was keeping in touch, but he didn't think it was a big deal and didn't feel like there was a need to completely cut her out.

Eventually she did just stop contacting him and they drifted and they haven't talked since then. This is the only kind of weird thing that's happened with him in relation to cheating but I feel like almost traumatized by it. It's been 10 months since this stuff happened, she's not our friend, we never run into her, but every few weeks or so I'll suddenly remember this happening and feel really triggered and feel like he's already cheated on me. And even worse, I've started feeling like he is just some kind of sleazy guy who will cheat on me in the future, and I'll tense up whenever he talks to another girl or even if we're in public around other women, I'll feel like he must be looking at them and checking them out. It's gotten to the point where everything will be going fine but I'll suddenly remember the thing that happened with the hookup, or feel like he's looking at another girl, and suddenly start panicking and feel like he's going to cheat. It sucks because he's a really friendly and confident person, so anytime he talks to a girl I think is prettier than me, I'll start getting upset.

I know how crazy all this sounds. I've talked to close friends about it and they consistently tell me that I have nothing to worry about and that I've found a really great guy. I believe this on most days, he's really loyal, attentive, and caring. But I feel like I can't shake this perception that I had of him when we first started dating. It's not fair to him. Honestly, I'm not the type of person to casually hook up with anyone so I think that's part of it too, that I think he must just be sleazy because he did that. I KNOW that's ridiculous and lots of people have platonic hook ups. 

I'd really appreciate if anyone has any suggestions about how I can move past this or share their experiences with thoughts like these. Thank you &lt;3