How do I stop feeling this way about my girlfriend?

I'll try to make this brief...in early February, my girlfriend met a guy at a bar and developed feelings for him. She broke up with me that week because she was confused about her feelings. We were broken up for about three weeks, and during those three weeks she slept with this other guy (twice). Then we hung out and she slept with me. When she slept with me, I was not aware that she was sleeping with another guy. In fact, I didn't even know this other guy existed. When she broke up with me, she only said it was because she was "confused". I asked if there was anyone else and she told me there wasn't. She said she still loved me but she was just confused about her feelings and wanted to be single for a while. So I was pretty upset and felt betrayed when I found out later on that there was another guy involved...even though she technically never cheated on me.

Anyways, she eventually realized that leaving me was a horrible decision and we got back together about three weeks ago. She says she made a horrible mistake and that she wants to be with me forever. However, lately I've been thinking a lot about this other guy that she slept with. It's on my mind a lot and that makes me feel pretty upset...it gives me that sick feeling in my stomach that I got when she broke up with me. So, my question is, what can I do to stop thinking about this? I talked to her about my feelings, and while that helped a little, she got a bit upset and said she doesn't want me to bring up the past anymore because it makes her feel bad. I do believe she regrets the situation and feels awful about it, but my mind still wanders and I can't help but think about the past. For example, when we have sex I'll think about her having sex with this other guy and that totally messes with my head. I wish these thoughts would go away. Any help would be greatly appreciated!

This is something you either accept and force it out of your head, or you split with her and find someone else.
If you don't, then this will eat away at you, you'll be resentful towards her and the relationship will fall apart after lots of fighting and arguing.

I don't know of any magic cure, when i found out my (now ex) gf slept with someone when we were on a break that was it for me, end of any hope of reconcilliation. (Turned out for the best though :D)

So, good luck!