I'm sorry, I think almost all of us have been there. It is her fault for not trying to work it out, you don't want to be with someone that just runs away at the first sign of a problem.
Right now I think you need some time to yourself. Do something other than thinking about it, as hard as that is. Your not going to get anything out of thinking about why she did it, we will never understand why women do the things they do. And Im sure some of them will never understand men, guys do just as much shitty stuff to women. But men are more predictable, women tend to hide that inner evil until later on down the road.
My ex came back claiming that she got scared about how close we were getting and she was sorry about what she did. She wanted me to forgive her and went into this sob story about how she didn't want me to hurt her and had never had anyone know her like I did. But you know what, as much as I care about her, those are just words....nothing more. She left me when I needed her most. I was on a trial of medications for insomnia and was feeling kinda emotional sometimes, school was wrapping up and I had to get some college application in quickly, and I was getting ready to move out of the house. I was stressed out and she just added to that by giving me a very lame excuse for putting our relationship on hold. I know I was closer to her than anyone had been before, and I can even understand her getting scared, but thats no excuse for what she did...and then what started happening months after that, but thats another story all together.
I mean you think about the sweet moments like how we spent 8 hours straight on the phone during Thanksgiving because we couldn't be together, or going to the movies, or going out to lunch. But for some reason 1000 amazing and wonderful memories can be overshadowed by 1 painful experience. You remember pain and hurt a lot easier than pleasure, at least in my case.
I can forgive, but I sure as hell don't forget. Thats just the way I live my life.