Bruises from angry boyfriend. What level of anger is normal?

My boyfriend is constantly breaking up with me every time we fight. I feel like this is some sort of manipulation in order to make me tell him I want him to stay.
More recently when we fight he's grabbed my arms so hard there are huge scary looking bruises, he's twisted my arm to the point where I've screamed that he will break it. He's spit on me, pushed me and called me numerous horrid names.
I know that a lot of the time when we fight I don't give him much of a break, mostly because he has a ridiculously hard time apologizing for anything he does and ends up making far-fetched excuses for all his behaviors.
I don't know what to do anymore. He promises me he will get help, he will change, that this will stop. I just don't know if I believe it anymore. I also have no idea how to get help. Or where to get help. Or what level of anger is even normal. He will punish himself in these outbursts as well slamming his head into walls and clawing his arms.
Please help.