I've felt this way before, and on occasion I still do.
What has helped me overcome my feelings of worthlessness is to set some goal that I wish to achieve, not something easy or frivolous, but something that has meaning to me.
If you've had a dream to accomplish something fantastic or acquire something you really want, determine what steps you need to take to achieve it.
Most times, people give up on their dreams because the first steps they take are so overwhelming that they become frustrated and depressed.
Frustration is a sign that your challenging yourself.
Break down that tough step into smaller steps.
"Corny Cliche Alert"
It's like video games (if your into them), you don't gain a full bar of exp all at once, you slowly fill smaller bars (small goals) and eventually you level up (big goal).
Hope this long winded suggestion helps.
P.S.
The occasions where I still get these feelings comes when I sit idle, or blast off into daydream world (I have a horrible case of ADD), where I think of all the things that I did wrong in the past, how bad the problems are of my current situation are, or how much I'd rather be doing like: playing video games, sleeping, smoking pot, or anything else easier than what I'm doing now..
In order to regain myself, I prioritize what Needs to be done, not what I Want right now for immediate satisfaction, and those feelings usually calmly disappear. (I do this alot)
By the end of the day, all I have to look back on are all the things I worked on and accomplished towards reaching my goal, instead of how much I hate the situation my life is in (which I catch myself saying, but it's more like I HATE MY LIFE).
P.S.S
Finding something your good at (fixing things, talking to difficult people, mathmatics, cooking) and using it to help someone who isn't good at what you do in exchange for something they can help you with, can also really help to free you from your shackles, and give you a sense of fulfullment and worh.
P.S.S.
Be Happy
It can always be worse =P