Should this upset me?

Before I start, I have a mental illness called borderline personality disorder (no I don't have a split personality) it's an attachment disorder, so I get very attached to people and I constantly look for ways they're hurting me. So I pretty much make problems out of nothing. I know I'm crazy and I really do my best but there's not much I can do when I lose myself.

A big part of this is me getting upset over unreasonable things and sometimes I can't tell if I'm being unfair or not so I need some opinions.

Me and my boyfriend met in February last year and I instantly developed feelings for him (attachment disorder) and he took about a month to come around to me. He asked a girl out that he works with "to see if they wanted a relationship together" but also to try to distract himself from me cause I was 15 and he was 19. I asked him not to go on this date with this girl cause it would make me sad. He didn't go on the date and me and him stared dating a few weeks later.

He is still in contact with this girl (her name is Marisa) and I really really don't like it. He says that the fact that he asked her out doesn't mean anything, that it was just to see if they liked each other and they never found out bla bla bla stuff like that.

Am I being reasonable? Or is it my illness?