Kiss of Death?

Hello all, first time poster here, looking for some advice about the situation I have found myself in, so good, bad and indifferent all will be appreciated. First of all I'll say from the offset that I'm no saint but never have claimed to be and have taken steps to ensure I clean up my act, which I have been successful at doing so. Okay to start. I've been seeing this girl now about a year and a half. We started January 2012 and by the start of April 2012 we were sleeping together every night (both of us being in college). We had agreed to be exclusive since late Feburary. At the end of April she kissed another guy. I found out through checking up on her facebook ( i know not great but it was more so out of boredom and missing her than actively looking for something like that) She messaged her friends about kissing this guy and even linked her friends his Facebook page. The next day there was a message from her giving him her phone number. From reading her message there didn't seem to be any hint of remorse as she flaunted her conquest to her friends. She even said she had a great night best night in ages (it was her good friends birthday, so not sure is that in relation to her birthday or the guy she was with). Anyway that was at the weekend and the next night she would have been back in my bed again. I can remember her telling me she had such a great night and showing me photos which included pictures of them posing together (nothing on the face of it to be worried about). That was nearly a year ago. I found out this a couple of weeks ago. When i asked her about it she denied it to the last, until i posted the messages she had sent between her friends. Even then she claimed that we wern't exclusive until i pin-pointed the message where she said not to tell me or any of my friends. She has no defense as to what happened and just said she was drunk. Now to the part about me not being a saint. I was away for 3 months from June to September travelling and kissed two girls out there. I broke down the next day to her and told her everything. I literally cried myself to sleep the following couple of nights after this happened. She gave me so much abuse (and rightly so) but never let her little secret out. I felt so low after what happened and how i broke her heart. Now since then i have stopped drinking, as this is my downfall. I just cant trust myself 100 percent when i drink, so i don't anymore. I feel though the fact she gave me such abuse while never letting on what she did herself gut wrenching. When I asked her, she just said that she did not see what she did as a big deal. Now that I have found out, i don't know what to do. Yes i strayed and she forgave me, but I broke down and told her everything and after that took steps to ensure i never ever would do that again. She says she is sorry now for what she has done, and cries herself to sleep and that. But really i think its over the fact that I may leave her rather than the act itself. Also she continues to go out drinking and partying regularly. She is way now travelling for another month and a half and I really am not sure of where our relationship will go from now. I just don't know can i trust her anymore. Like she claims that it was only the one time, but the way she denied it to the last makes me feel like there could be other times i may never hear of. Any advice, questions etc lay it to me as I'm at a cross roads as to whether I should leave her go and get on with my life or give it another shot.

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