I had manic depression from about the age of 12 to 20, I was put on doses of Prozac but non of them seemed to make any change.
I cant suggest any magic cures or any particular ways of coping with it, all I can say is it won't last forever. You are still in your teens, your hormones are still kinda all over the place. Thats what I put my depression down to as I didn't have anything stressful in my life to be depressed about so I can only think that my hormones were acting up. As soon as I hit 20 (or around that age) it was like a switch, suddenly I wasn't sad and infact, since then I have been getting happier and happier and it has now left me in the giddy state I am in now.
That's pretty much my advice, just to remember that there is something to look forward to, I don't take much seriously now, life is one big playground and who cares if you make errors or mistakes, as long as your having fun that's what counts. I remember when I was too anxious to even go an buy an ice cream from a van or catch a bus into the next town, now I am fine to catch flights around the world on my own and ask complete strangers stupid questions, life may seem scary, but it's really not.