I, too, was diagnosed with Manic Depression, however I'm unclear as to weather or not that diagnoses was accurate. That being said, anything that I relate here is my personal experience and may very well have no bearing at all on your current situation. Secondly, I'm extremely sorry to hear about what you're going through I've been there and it's not a fun place to be at all
What finally worked for me to eliminate (and by eliminate I mean totally gone - no sleepless nights (unless I've had too much Caffeine), no periods of sadness for no reason for at least the last 7 years or so) my depression was a change of mind. For me, my perspective on the world, the Universe, and human nature was the cause. It took me a long time to realize that. I was spending an enormous amount of time focusing on things that were "not my problem" and because I couldn't solve them I was getting depressed. Mind you, it was incredibly subtle - it took me years to discover what was actually happening. I had to force myself to not focus on how other people were wrong, and focus instead on how I was right. It was a matter of "Positive Thinking" (I know, sounds like a 12 step program, huh?) but it did actually work for me.
I found hobbies - got involved with Theater (which became my #1 love (for entertainment) and pushed music to #2). I began socializing more, which helped alot as well.
One thing that I can suggest (at least to start) is to understand on a chemical level how your body engages in emotion. An extremely good explanation is the movie "What the Bleep Do You Know?". The acting is lousy but there is documentary footage in there that is quite interesting. Despite the acting, the total package makes it well worth the watch
I have found that being happy becomes a habit. Generally I'm an extremely enjoyable person to be around - I make people laugh, which makes me feel happy as well. It's contagious! However, as I said, it *is* a habit - and it was an extremely difficult one to start.
I hope this helps...if you have any questions or if you need anything, don't hesitate to contact me here (or in PM if you'd like). I don't like seeing people go through this sort of thing - the lack of a "reason" I think is what affected me the most. You can't solve the problem if all you see are symptoms.