Hey, AC - go get help, get medicated and use the time you are medicated to do CBT (because the combination of the two has a good success rate), start jogging (because annoyingly, this too has a good success rate), eat more healthily (specific benefits of this are, I believe, a bit more contentious, but cooking properly is a great and positive activity irregardless), and while you're at it, identify what in your life and yourself you need to change to protect yourself from being depressed. Then use that to actually make the changes - this process took me about five years, but became progressively more worth it and easier. There's no magic bullet, it is hard work, and if you are susceptible to depression you probably need to keep at it in a small way forever.
This doesn't work for everybody (some people do seem to just have bad chemistry), and really isn't easy, but it did for me.
Perhaps the hardest bit is actually getting help in the first place, it took me months and the damage to my life was pretty extensive. Then one day I had a breakdown and sat weeping on my kitchen floor, because I couldn't cope with choosing between frozen pies for dinner and thought "Shit, I can't fix this by myself.". A mere three weeks later I'd actually gone to one of the several doctor's appointments I made.
Anyway, I'm rambling. Don't spam your brain - do something about the sadness and pain.