I went through a serious period of depression during high school. I missed a huge amount of school and there was a period of about a month during which my parents were considering having me committed to an institution.

The key to beating depression is introspection. Recognize it for what it is: a couple chemicals in your brain that are out of whack. No small matter, and there's probably a cause. But your mental faculties are still there -- they just have a distraction breaking their concentration. Think of it just like a loud noise, and tune it out, then figure out where it's coming from and silence it.