I can't stop thinking about my boyfriend with other girls!?

So last night I did something really stupid: I looked through my boyfriend's Facebook messages while he was at work. We've been together for almost a year now, and I do trust him, but it was just too tempting to take a look. He hasn't been talking to any girls after we became a couple (other than work or school related stuff), but last automn he was talking to like 10 different girls. One of them I understood had spent the night at his place just a couple weeks before we met. I know this was before we met so it shouldn't matter but it just makes me crazy to think about him with someone else! Another girl (she's now dating one of his room mates, so i see her all the time) seemed reeeally into him, turning all conversations towards sex. What makes me sad is that both him and I hate this girl, she's always trying to make a fool out of people and really annoying. But my bf seemed to enjoy talking to her and invited her over to watch a movie. The thought of her sitting with his arms around her makes me feel sick. I admitted what i had done and talked to him about it, he said he didn't kiss her and didn't have sex with any of the girls. then I asked when te last time he had sex before me was, then he answered 4 months before with his ex (they were dating 2 years earlier but she cheated on him and he ended it). Now i just see these images in my head of them having sex in our bed. What should I do? How can I stop having these thoughts?
I can't stop thinking about my boyfriend with other girls!?
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