How can I get over my boyfriend's past (relationships)?

First of all, I realize how immature it is to be hung up on my boyfriends past girlfriends. But I do tend to focus on these things and then I get paranoid. We have been together for 2+ years and I don’t want stupid things to jeopardize what we have. He has never cheated on me (and I believe he never would). He is still friendly with his ex girlfriends and that is what bothers me, even though I know it’s petty and stupid. I have borderline personality disorder and I am constantly fearing being abandoned by anyone, especially him, and my mind always goes to worst case scenario and I take it out on him, and then myself. How do I STOP doing this?