Jealousy

I trust my boyfriend, I do. I'm so confident in our relationship and his loyalty toward me and most importantly his love for me. But I can't help but think about the girls of his past and I will sit and analyze every piece of their relationships (mostly physical) until I'm sick. How do I get past this? It's not only affecting me but our relationship as well because I constantly bring it up to him and he just doesn't know how to ease my mind anymore. I can't help but picture him with someone else and I can't stand it!!