Help anyone please, its long but i disparately need help.?

There has been a constant battle going on involving men and my best friend. I met her about 4 years ago and I've been in love ever since. We have talked literally every single day for the past 4 years, even while she was studying abroad in Australia. While she was abroad last semester she kept saying that when she returned she wanted to by my girlfriend and i really want to be her boyfriend. Unfortunately upon returning she started dating some other guy. When i got upset about it she said that her relationship with him was nothing serious, and when she dated me she wanted it to be serious, and she was worried if we dated now we would screw it up too early. All she asked is that I remain patient, so I did. over the past few months they have been fighting and every time they fight she breaks up with him and comes running to me. Then they make up and I'm left in the dust, when i give her a hard time about it she gets mad at me for trying to interfere. In a more recent fight of there's he threatened her life, and got very abusive. Not physically but verbally insulted her, ripped apart her bed room and yelled and made a lot of threats. This happened the day before thanksgiving break. Throughout break we were together every day and she insisted that she wanted to be done with him, and upon returning to school it would be over. When she returned to school however she ended up getting back together with him. And this time she wasn't allowed to talk to me anymore. He doesn't like me because were so close, and that I'm always trying to convince her to dump him, plus he found out we had a few times. So she stopped talking to me for 2 weeks. I was ready to be over her when she came crying back to me apologizing saying she wants nothing more to do with him, she said hes too controlling and everyone hates her for talking to him still. Even her parents wouldn't talk to her because of it. I told her that if i forgave her and pretended she hadn't previously hurt me, then she has to promise she would be finished with this guy entirely, no more communication of any kind and she agreed. She told him for a few days to leaver her alone and they they shouldn't be seeing each other anymore. Then a few days later i found out they were hanging out again. When i confronted her about it she said that he makes her happy and she wants to continue to hangout with him for the rest of this week. Were leaving on Friday for winter break and she says she is cutter herself loose of him over break and letting it end then, however she wants the next few days to continue to see him. I flipped and told her not to talk to me anymore. She thinks I'm being irrational and i don't have a right to be mad, however I think my argument is justified. The way I see it is no matter how many people want her apart from him, one month wont make any difference if she doesnt want to be apart from him. I don't know if i should continue to hang out with her when we go home for winter break. I know she wont see him for the next month cause he lives far away from us and her parents wont allow her to see him, but i cant help but feel like im going to end up getting hurt again. I cant allow myself to become vulnerable to her with the risk of her screwing me over when we return to school next semester. Am i acting irrationally? should i give her a chance over winter break, or just cut her lose now? I feel like i should but i cant see her ever being serious about me so long as she likes this other guy. Any advice would be greatly appreciated. Please help!!!!