*Pllleeease help me.. this is serious!*

My boyfriend, Zac, and I have been together for over eight months now and I'm very much in love with him, but for the past three months or so he's been treating me like crap. Just calling me names for no reason, never wanting to be with me, always choosing to be with his friends over me, ect... We only get to see eachother about once or twice a week and we only live two miles away from eachother. Every time I finally get the courage to break up with him, he starts being really nice to me, all of a sudden. Then I can't get myself to break up with him. Well...just recently..within the past couple weeks, I started "talking" to my ex boyfriend, Luke, again. He's the nicest, most caring guy I've ever met! The reason why we broke up in the past, is because I sort of liked someone else. Needless to say breaking up with Luke was a big mistake, because I've still liked him ever since we broke up...almost a year ago. Well...now Zac is starting to be very very nice to me, so I'm very happy about that, but the problem is...I like Luke so much and I know I wouldn't ever have to worry about him being mean to me, but I know Zac being nice to me won't last long...so what should I do? I'm so lost! I can't just get rid of Zac for no reason, even though I know he'll just keep treating me bad. And I want to be with Luke so bad. It's messing up everything, because when I'm with Zac...I'm always thinking about Luke and when I'm talking to Luke...I'm always thinking about Zac. HELP! ASAP!