OP, I'm in a very similar situation as you. I'm heading off to college in the fall myself, and I intend to just get the hell away from my mom, because she seems to have no interest in being a real mother to me.

You, however, do have a family that you are close with. If you're close with any of your aunts or uncles on her side, maybe they could help you. I'd figure that she'd be more likely to listen to her siblings then either you or some random psychologist. Also, the next time she physically hurts you, call the cops. I know that it'll be immensely difficult for you, but it may be the only way to get her any help if she is indeed undiagnosed bipolar or somesuch. She doesn't sound like the kind of person that would either get help on her own or would be persuaded to get help.

But something strikes me as odd, OP. You said that "we're affectionate when we're not fighting," but how often is that and how sincere is the affection? Just trying to figure out the situation.

Anyhow, I think that going off to college will be a good thing for both of you, since you'll have some distance between you and won't have to see each other everyday.

Let us know how it goes, OP.