Let it go! Every time you have a thought about it, chase the thought away. Don't think about it, don't dwell on it. Let it go.
If you don't you will drive her away and every time you fight you'll bring it up again and she'll get sick of it and leave. That's what my ex did to me and is a big part of the reason I left.
By the way, I agree with Risa. Don't post in a public forum and then tell people to **** off because you don't like what they have to say. My honest opinion is that once a couple has broken up, getting back together is a bad idea. More often than not whatever drove them apart the first time hasn't been dealt with and they end up back where they were but worse. My actual advice would be get out now, but since you asked how to get over her sleeping with someone else, I advise to simply let it go. I somehow doubt you'll be able to though. My prediction, you will let this keep gnawing at you like a dog with a bone, it will start seriously affecting your mood, you will become jealous and possessive. Your gf will get sick of that and will either just get up and leave, or she will cheat on you. That's my prediction.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.