Overweight, and need help loosing it all?

I've been overweight since I was about 4 or 5. I exercise and eat right, but then I just give up. I don't know why it's just hard for me to keep exercising and dieting. I really want to lose weight, because being fat makes me VERY self-concious. I have a very slow metabolism. Does anyone have any advice that will help someone stay motivated and any advice to speed up my metablosim? I've tried everything from Weight Watchers to the Atkins diet , to being bulimic. I've tried putting pictures up of skinny girls and i've tried listening to music while i work out and NOTHING seems to work..