CityOfAngels

I didn't know which board to post this is, so I chose Sexuality since this situation has a bit to do with sex...

I've been officially with my girlfriend since May 5th, 2003, but we started off in March as just fuck-buddies. She and I both agreed that we were both horny, but she's going to go off to UC Davis in September so we decided to not make our relationship any more than just sex. We had also made a pact to not have sex with someone else without telling each other about it either first or right afterwards.
Time went by. I've asked her countless times, "Who have you had sex with?" She says, "My ex-boyfriend whom I was with for a year, and some guy I cheated on him with while I was drunk, which was only once." I asked, "Were any of those times when you were with me?" She said, "No."
For some reason she wanted to drop me off on her friend because I guess she was getting too attached to me and noticed that if she didn't let me go, it would hurt too much when she left. For about a month I kept declining; "No! All I want is you!" Little did she know that I was very fond of her myself. So, after all the pressure from both her and her friend, I finally gave up and fucked her friend. The day afterwards, I called her up and said, "Finally what you wanted to happen has happened." She was devastated. After two weeks of not talking to me, she finally showed up at my door and told me how she had fallen in love with me. In truth, I was already in love with her, and the only reason I fucked the other girl (other than she was hot as hell) was because I thought that it was what she wanted. Apparently it wasn't. After a grip of drama, I asked her out on May 5, 2003, and she said yes.
Despite the bad memories of the other girl and I, my girlfriend and I have been happily together. We've had our ups and downs (especially with her habitual lying), but overall we knew that we love each other with our entire beings. But then this morning, she was talking about getting married, and I told her, "How can I marry a girl who lies to me and keeps secrets from me? How can I call a woman my other half when she won't even tell me about her life?" So she began telling me, one after the other, explanations of her feelings of different things. That conversation went great. It was awesome to finally see some honesty. But then later on, she said, "Oh yeah...there's one more thing I forgot to mention. Two days after I took your virginity, my ex-boyfriend," (not the one she was with for a year, but one she was with for a month just before me),"and I fucked." I was like, "why didn't you tell me before!?" and she said, "I just forgot!" BULLSHIT! How can you forget that you have sex with someone? I asked her if she was drunk or high and she said, "No."
I don't know what to do. I feel so damned betrayed. She not only fucked someone else without telling me right afterwards, but it's like, all this time we've been "In love" and she just now tells me this. Plus she lied to me when she said that the only guys were that one ex-boyfriend and the other guy she cheated on him with. I don't even know if anything else is gonna pop up that she "forgot." What should I do? Should I dump her or give her another chance? I really love her, but I'm afraid my love for her might be blinding me to the real truth of how deliterious our relationship is to me. Please help!