I have a short temper and need help asap!?

My short temper is horrible! I always want to cry, hit something or someone, I take it out on the world. My parents hate it, &I'm driving people away. Biggest current issue is babysitting. The boys are great but one, 10 year old, is clinically bipolar &when he has an out burst I can usually put up with it but today I'm losing it. I know its not his fault but I don't know how to calm down because I always let him off and today is just not one of those days! I'm ignoring all of them because of him and its not right but if I go out of the room and see him I'm going to anger up quickly again. I need quick help.