Why does she behaves like that and why I am so stupid?

Anyways, the story goes like that: I broke up with my gf of 3 years about a month ago, I came home earlier, and she was walking whith some other guy, I got shouted at why I'm coming home early etc... She was behaving weird, so I asked her about him, since I got jelly..we got in a fight, so I packed my stuf and go home to my old man...
The same night we made an agreement to have a break for a week to calm down and have a talk...After 3 or 4 days she called me and said she's alright we broke up, which I wasn't alright, because we had an alright relationship, got through so many important thing during that time and I didn't want it to end in such way, plus all I did on and off work was thinking about her and thinking what could I do better...
So after few days I called her again asked if she changed her mind, which she didn't, plus she wouldn't tell me why, I got really depressive, so I did slightly wrong move, I went to her facebook to see if she chatted with her friends about the issue and if she told them what she's thinking about all this... But what I found out she was already dating that guy,started drinking and going out which she never really did before.
Out of desperation I drove to her at 4 Am :D kneeling and crying, and she was the rudest person I've ever seen. And everytime afterwards when I called her she only manage shouting at me and being rude, plus she wouldn't tell me any truth (I didn't asked her about partying and the guy at first, well she told me when I asked her what was she doing, that she was just sitting and home, shopping...l

So the questions are:
1. why on earth she behaves like that I never existed, cause she was singlemost nicest person before. Could it be guilt?
2. Could the cheating be caused by troubled childhood? (no dad, and she was abused)
3. How could she move on so quick?
4. Why do I miss her so bloody much, (including all the other emotions, except for happines), I mean i know she traited our love, and that traitors shouldn't be forgiven. But I still want her with all her flaws, I would forgive her for the cheating. But she would listen...

I just hope some had similar situation or at least clearer insight than me, cause I became a bloody wreck in last month..
Ps: hope you understood my english..not my native language