In a certain sense, being 20 is a good thing. It means you still have time to try things out and figure out what you're interested in. You don't need to work at the same pace as everyone else and you don't need to have some kind of grand life plan. Not accomplishing anything in life is impossible (we do accomplish things all the time!), but that's not to say that having to accomplish something in life is the only thing worth living for. You don't have to be great at anything. You're a valuable person whether you've accomplished something, or not. Always remember that!
I'm not expert, but my advice would be to just explore a bit, look at apprenticeships, look up night-classes, stuff like that, or make an appointment with a careers counsellor. They're experts at finding out what kind of jobs suit people. Take up a hobbie. For me, karate boosts my self-confidence, because every time I accomplish something I thought I couldn't do before, it makes me feel good about myself. It gives more purpose to my week.
I'm sad that you're family isn't with you on this. Have you tried talking to them? Maybe you have to remind them that you're a bit lost at the moment and need their love.
Forging an iron signature...