How do i get out of this?

My bf of a year and a half met at university. He was suprised when I told him I liked him b4 we started dating. I excel at school, am well known, good-looking and have a job w/ a Professor. As for him he flunked out and works a retail job now at 26 years old. We usually get along but he never understands anything (from my cles to PMS). He freaks out at everything and makes me feel badly. He even swears at me for no reason and takes his stress out on me. Hes bringing me down. I dunno how I can get out of it b/c I always wait around to see if things will change and they do for a few weeks. And I explain things over and over that Im going through and ask him to support me but he comes to his own wild conclusions. Oh and Ive broken up w/ him last spring and he got really freaky and creepy....so im scared he wont leave me alone again if i walk away.