Hi,
I read once something like "as you think in your heart so you are", i can't quite remember the quote but the meaning of it just shows that if you tell yourself you are worthless then you will be like that in your activity. How about every day when you get up, look at your self in the mirror and just repeat to yourself that you are full of worth! Pick a feature, any thing and focus on that. Don't think about all the negative things, no one is perfect and no one expects you to be so. Think of anything you have ever achieved, it can be something so simple like making a sandwich, literally anything! just focus on that, it was a completed task, you can do something!
Also, if you feel as though you can not complete tasks, can you try breaking them down into small small chunks, then when you have completed each small bit you will be able to build up your confidence more and more as you go.