Sorry to hear that you are feeling so down right now, I know it's never fun to be feeling that way.
As you may know, I deal with some similar issues, and while I am still struggling, I truly feel like I am getting closer to finding the things that will work for me.
One of those things is meditation.
I was reading the book Eat, Pray, Love (great book!) and the author talks about realizing how boring one's mind truly is, just the same thoughts over, and over and over again, and i realized that I am definitely guilty of that one. I get into a thought pattern that starts out seeming innocent enough, but if left unchecked, wham! So, as Christine Kane recommends over on her blog, I am learning to "cultivate the observer". I used to try to repress (surfing the forums, eating, etc), but that would always come back and bite me in the butt. So this cultivating thing, I just kind of acknowledge and say No.
I have also realized that for me, when I start to listen to the "I'm worthless" gremlin, the gremlin wants me isolated and alone, cut off from my Source. A quick fix can be doing something like going to a Yoga class. We always start with a meditation and an intention. That reminder plus the social interaction work wonders for me.
The last thing that has been helping me, is Gratitude. I've begun collecting things like letters from friends, emails, photos, inspirational quotes, etc and put them into a place where I can go to for a daily reminder of how good life is, even if it is sometimes painful.
I hope that you can find something useful in this post, and that you find a way out of your sadness.