It's possible that you may be suffering from Depression. It's nothing to feel bad about, a lot of people go through it at some point in their life. You may also just be experiencing a hormone related issue. It is quite common for teenage girls to obtain a hormone imbalance while their hormones are still developing and changing. Hormone Imbalances can cause depression and anxiety-like symptoms. I'd talk to a doctor about it and have your hormones checked. I also agree with bricheese. Hobbies and social activities will help you take your mind off of it. Also when you're feeling particularly down, I'd suggest listening to some happy and up-beat music, like techno, nightcore, dubstep, or electronica.