Remember that you are worth so much that God took the form of a man, and died an incredibly painful death in order to redeem you from all that seperates you from Him.
But if you're looking for something a little more material, try taking up a hobby and learning it to at least a proficient level. Also, if you're still in school, working hard to pass the grade often gives great feelings of satisfaction with oneself. Really, just find something that makes you feel accomplished with yourself, and produces some productivity and good habits along the side. It'll change you inside and out, for the better.