I've just finished reading Mastering Your Mean Girl by self-help guru, Melissa Ambrosini. Let's face it, we've all got a mean girl, that nasty voice in your head that tells you you're not good enough, not pretty enough, not worthy enough... In her book, Melissa explains how you can defeat your inner mean girl. She says that at any moment you can choose love or fear. For example, 
LOVE - I exercise because I love my body; or
FEAR - I exercise because I hate the size of my bum and thighs.
Throughout the book there are opportunities for you to write down actions you can take to change the way you think.  It is not specifically written for teen girls but it would be really useful to know this stuff while you're in your teens. I wish I'd known this stuff in high school. You can visit her website here.