The best thing I can suggest is to find something you enjoy doing, preferrably a few things you enjoy, and make them part of your life. Quit worrying about other people's standards because you can never live up to what everyone else things you *should* be doing or thinking.
My Dad suffers from mild depression... Doesn't like to plan vacations or travel but enjoys himself if someone else does. Is perfectly happy spending the day reading the paper, checking some internet stuff, listening to music, and maybe walking to the store for groceries and stuff. Every once in a while, he feels the motivation to go do something and he goes and does it. But when he doesn't feel like doing anything, he falls back on whatever it is that he DOES feel like doing, and to hell with what anyone thinks.
Shed negativity. Seriously, if there is someone or something in your life that's dragging you down, avoidance may help. You *should* be doing whatever the hell it is you want to do, as long as it's not destructive, illegal, or non-sustaining from a well-being point of view. For example, even if you don't like your job you can't just quit unless you find another one, you can't quit going to the grocery store just because you hate standing in line at the checkout, etc. But if a "friend" keeps bagging on you for doing or not doing something, or not being a certain way, then they're not a friend and you need to hang with someone else.
Fitness... I feel better after exercising and I feel better when I'm in good shape. Every time I work out it hurts like hell because of a not totally successful back surgery, but if I don't work out then it'll be worse and it would hurt all the time. If you feel sore for no reason, it's your body telling you that you're so out of shape that even daily life is too strenuous. So go put some physical stress on your body in a healthy way, and build back up some natural resistance to the world in general.
If necessary, pay to go to a fitness center and get a personal trainer, if you simply can't get started or don't know how to make fitness part of your lifestyle. Once you get into the good habits, you'll feel better all the time for no reason other than you're healthy.
Smokers feel that way just from lighting up and smoking is a proven weight-loss technique, but the downside of cancer is a pretty big deterrent in my book so I don't suggest trying that.