I'm a fifteen-year-old girl, too. Hah. I have the same problem, for the most part - but I don't see it as much of a problem as it is a habit. It's not a very bad habit, either. Sexuality is normal for us, and as long as it's not painful - unless you like that kinda' thing - or
harmful, I should really say, then don't feel bad about it. A great majority of females masturbate, and read/watch pornography. Don't get so bent out of shape about it. To feel guilty is normal. Hell, I still feel guilty sometimes, but it's just an immediate reflex to what i've done and what the older generation has to say about it.
As long as you're not hurting yourself or anyone else, then embrace the pleasure your body can produce for ya' as much as you like. 'least, that's what I do. Keehee.