Please remember that it's not your fault. I don't have a whole lot to complain about either—I'm actually a pretty lucky guy in the grand scheme of things—but in some ways, that makes it even worse. It's easy to blame yourself for the way you're feeling if there isn't anything obviously wrong with your life. Maybe you tell yourself you're just "spoiled" or "ungrateful"; maybe you feel like you have no business being unhappy (I know I do at times). Like you said though, it's not you—it's your brain. Have you talked to anybody about getting some prescription antidepressants? They can do a surprisingly good job of keeping these chemical imbalances in check, provided that your expectations aren't too high going in. Not the most helpful advice, I know, but at least you're not alone: I've been in a similar place and we're both still here.
Good luck getting through this rough patch, and I hope you feel better soon. Stay strong out there.
-L