Jealously

My boyfriend and I have been together for about two and a half years. We really love each other, and we've been through a lot. We're both pretty outgoing people and I have a lot of guy coworkers that I get along with. He knows they're my friends and he doesn't get jealous.. however he's currently enrolled in college and he keeps mentioning this same girl over and over. I love him to death and I want to trust him, but I feel very jealous. She's a very pretty girl and they have a lot in common. I just feel insecure and I realize that they're just friends but I can't help how I feel. How do I overcome this? I've never been the jealous type, and watching myself turn into this person makes me not even want to be in a relationship. He's never given me any reason to not believe him, and he feels like he can't even bring her up without me getting upset. Please give me some advice!