There are two keys: baby steps and positivity.
Don't try to tackle everything at once. Try something that makes you a little uncomfortable, and when you get used to that, move on to something bigger. That's what they do when someone is afraid of something. For spiders, deal with having a spider in a cage being in the same room as the person, then have the cage within a foot of them, and so on. Take it slow.
Being positive is huge. Telling yourself that you cannot do something is a sure way to make you unable to ever do it. Instead, try to tell yourself that getting a part-time job isn't that bad, that it's even easy. Don't automatically assume that it will be hard, even if that's your first instinct. Tell yourself that there are things you can do, even if you are a loser. Even losers can do some things.
That said, it's still very hard to convince yourself that you're not a worthless piece of shit. So . . . good luck?