My girlfriend cheated on me but she doesn't know I know she cheated. What should I do?

So I've gone out with this girl for about 2/3 weeks and I thought everything was going well until our last date, she seems a little bit spaced and nervous. So this past night, I had plans to go to a movie with her, her friend, and her friend's boyfriend. But I had to cancel because I had an unexpected visit from my god-parents, who I was then forced to eat dinner with, boring. So once I got home I went online and started talking to friends. Suddenly I got an IM from one of her friend's who she was sleeping over at her house, saying: Hi, umm please don't tell ---- that I told you, that she hooked up with this guy last night.

Her friend then asked if I still had feelings for her, I did, but after that how could I still like someone who cheated. And you know what's funny, some of her good friends that day were asking me if I was gonna get her something for V-Day sure it is more then a few weeks away but they were curious. I was to, I wanted to get her the most romantic, beautiful, amazing gift ever for I girl I was really falling for. I had a feeling something was wrong because the past few days she has been ignoring me, and not answering my texts or IMs, we usually talk everyday just making sure you know I'm not a stalker. Well back to the point. So what should I do? I still really like her, but I feel betrayed beyond imaginable, and how could I look into her eyes again and tell her I really like her? How!? It's funy because because once we started dating, her friends kept telling me to treat her right and do anything I needed for her to be happy, and I believed I did, because they told me she had a string of very horrible relationships in the past year, and she really liked me and hoped that our's could be amazing and go far. Now I am depressed and angry that I let this happen to myself. I am really dissapointed in her, especially when we first started dating, the first thing she asked me was:

If I was only pretending to like her to get a** and I was honest, No I really liked her and she liked me. I Need Help! What should I do?
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so we just got into a fight. it's funny she said that if anyone ever asked about us we were gonna say we are going out...so now she starts telling me we were never an item and that nothing ever happened. so that's it she's out of my life, and I feel good
My girlfriend cheated on me but she doesn't know I know she cheated. What should I do?
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