How do I lose twenty pounds? Help!!?

How do I lose twenty pounds? I'm fifteen. I stand at 5 feet and 10 inches and I feel so fat. I weigh 165 pounds. I cry everyday just because of my reflection in the mirror. I'm a bulimic. I started ever since last year when I liked a guy from school, but I wasn't "beautiful" enough for him and I just feel so sad and so sorry for myself. Please help me. Everyday I tell myself "I promise I'll diet today," but I always end up eating a lot. It's really depressing. I can't find the motivation and inspiration to lose weight. As of now, I want to lose 20 pounds for prom on February and this time, I want to be sure I lose weight. So to sum it all up, do you have any tips and advice on exercise and eating? Btw, if you have nothing nice to say, please don't bother answering. Have a nice day! -Cody