I've been in a serious relationship for the past year and half (my first real one, at 26) and I'm finding it hard to let go and simply trust him. 

He hasn't really done anything to make me suspicious of any thing, but that doesn't stop the feelings, which manifest both mentally and physically. If I find out he's been around another woman at all, I feel sick to my stomach until I have some sort of "confirmation" that she isn't a "threat." I know this is an incredibly unhealthy way of reacting. I want to stop, but I'm having a hard time pinning down exactly why I feel that way. I think I may be processing events in terms of "eventually he'll realize he doesn't want me," or "he's going to meet the REAL girl that he's going to love." When he tells me he loves me, or that he thinks I'm wonderful, my first thought is ALWAYS "you're lying" before I quash it. I'm not nearly as pretty, thin, or sociable as his past girlfriends, and while we balance each other out because we're so different, we also have very few topics that we come together all the way on. 

Recently, we were both invited to a friend of his' small party, at which a girl with whom he had a brief physical relationship in the past will be. I found this out maybe 2 days ago, and I have not been able to rid myself of that same feeling, nor have I been able to snap myself out of what I would describe as a partial dissociation. It's bad enough that I don't want to go, but I also don't want to NOT go either. Thinking of it makes me want to puke. 

I'm not sure how to handle these emotions anymore, and I don't want to endanger my relationship anymore than I already have with my persistent inability to trust him. It isn't fair to him, and it isn't healthy for either of us. 

He and I have had extensive conversations (and a number of arguments) about this before - I try to be as open as I can about my bpd, when invasive thoughts are bad I will talk to him about them. But this time I just can't find the way to say it, I don't know what I need from him. I don't know why I can't believe him when he says he loves me.

 Has anyone else felt this way? Is this something that I can talk myself through? At this point, further therapy isn't an option, though I have had DBT in the past. 