Why am i such a b*tch?

Hi, im mel, im almost 20 years of age, & for as long as i can remember i can be really mean & say really evil things to people, just over little things. Like for instance my boyfriend ive been with him for 3 years, & he knows i truelly have a great heart, but then if i get really upset i go into a black mood & say really cruel things, & then break up with him over crap, at the time i feel as if that is the only way & i cant understand why after a cigarette or a walk im usually back to normal & get emotional & beg for forgiveness... why do i keep doing it?? how can i stop this?? Is it some type of problem, or is my age?? im not sure!! Please help, save my relationship