lots of teenagers feel the same way you do. maybe try to stop worrying about looks so much and just develop yourself in other ways. get a new hobby, study hard and learn interesting things, go out and meet some new people, go to a music festival, play outside.......distract yourself from superficial things such as the media's interpretation of beauty, which is waaay off to what most people think is beautiful anyways.
if you are confident in your abilities/skills then you WILL be attractive. if all you can think about is being pretty, then you will seem less pretty to people, but instead will seem rather shallow and vain. if you forget about being pretty and just say "fuck it! I don't care what you all think!" then i assure you people will find you more attractive.
also, you're only 15. i felt ugly when i was 15 too. it took until i got into college to realize that i was never ugly the whole time...I was just comparing myself to other "pretty" girls and felt ugly because i didn't look anything like them.
eat healthy, exercise, bathe, and that's all there is to it

beauty comes from the inside out!
sorry this post is long, i am rambling...