Does anything else help you cope with the negative feelings? Exercise, journaling, art? Some people say doing something to help ground you helps with the urges to self harm - holding cold ice, snapping a rubber band on your wrist when you start to think about self harming etc. And some say getting out feelings through non harmful methods is helpful - punching a pillow, tearing a phone book apart etc. Don't know if any of these might work for you. I hope you find a way to get out your feelings in a healthier way. Try to have fun at prom.