I hit my boyfriend and i need to stop.?

We've been together 5 months and whenever I'm angry I hit and throw stuff at my my boyfriend. Its gotten so bad I've left bruises on myself from hitting him and actually accidentally hit smelf and gave myself a bloody nose. I apologize all the time for it and he says it okay because he's a big Guy and he can handle it. I've never been this way and I'm so terribly in love with him I feel ashamed that I do this too him. We are both bipolar but he's never hit me nor have I had the fear That he would. Has anyone else been through this? How can I stop and mend our relationship?