My friend, I know exactly what you are talking about. I went through the same thing with an ex and I'm going through the same thing with my current girlfriend. To people like us, love comes with jealousy. I have a feeling that if she would have told you about this before you guys fell in love with each other, it wouldn't be nearly as bad. That's the conclusion I drew from my own experience. The best thing would have been to leave it in the past.
But you know about it now. I had the same experience and it still haunts me. I can see my girlfriend and other guys messing around and it really messes with my head sometimes. Recently, I've been working very hard on it. The thing to do is just talk to your girlfriend, or someone else about it (this forum is a great start). I know it may not help to say this, but the past is the past. It was before she knew you. Everything in this world happens for a reason. It may have even led to you two meeting in the first place, as crazy as that sounds. Send me a PM if you ever need to because I can truly relate to this experience. You're not alone.