You are not alone in this. I too get insanely angry at times when I am usually the first to dive for cover at the sign of an argument. I can only speak for myself of course, but in my case I know that some of my anger comes from repressing my opinion.
All my life I have been stifled, as a child I was meant to be "seen and not heard" and as my opinion only came out in adolescence in what might well be called tantrums - I'd break things, doors, furniture, and I'd hurt myself I learned that voicing an opinion was dangerous.
Instead of learning to speak my mind in a calm manner I took the message "You shouldn't speak, you're thoughts are stupid, you shouldn't ever have an opinion." So now I find that if I have an opinion that differs from others I think that I am wrong. Wrong = stupid, stupidity = anger, anger =self harm.... and you can imagine where the vicious circle leads for me.
Do you have any means to get rid of the anger? Vigorous exercise, or a really long walk helps me. SOme people recommend tearing up a really thick book - the yellow pages is good for that. Does writing it down help you? Screaming along to your favourite pissed off song? Finding an outlet for your anger is critical until you are in a place where you can work on why you were getting angry in the first place.
I'm around for a bit it you want to shout and scream on here.
You are braver than you think, and stronger than you know.