What is this fat under my breasts?

Im not sure if it is fat or skin, but its very annoying. Right under my bra, i have a roll of something that no matter what, it will not go away. Ive tried all the exercises but its still there. What is it? Does anyone else have this problem? How can i get rid of it? I'm only 17 so i cannot get surgery or anything. I would like to get rid of it naturally. Thanks!