I know you're a sweet, lovely person...YOU have to believe it, too. Seems like you want to help people; ever think of volunteering? I know it's a stretch when you are young...it's party time! But life can be long and it's not too early to think about what you can do that will satisfy you just doing it, getting lost in it, whether it be creating art, acting out politically, helping the disadvantaged, discovering the cure for cancer...ok, not necessary to make such a huge mark!
Just treat yourself like you would any other worthy person and soon you will convince yourself. Sometimes you have to pretend you are a new, improved version of yourself to throw the old hang-ups aside.