First thing you need to do is calm down. If you're this frazzled you'll make mountain out of a mole hill. Second you need to leave the past in the past and forgive and forget, don't hold grudges and let your friends be friends with whoever they want even if it's your worst enemy. Before you get angry, think about how petulant the argument you're having really is, and at your age I highly doubt the things you argue about are important at all.
Just chill and get lots of hugs. here's a freebie. =]