Why do i have such a breathy voice?!?

Since your only 15, it could be possible, though not likely. Having a breathy voice has it's advantages, your too young to remember "Jessica Rabbit" from "Who Framed Roger Rabbit", played by Kathleen Turner. It is major sexy! I would also enlist the help of a voice coach; this may help and also can boost your self esteem. I hope this helps!