I relate to not having confidence in anything. But unlike the other posters, accomplishing things does nothing to help. Not overall or in the specific thing I've accomplished things in.

For example, I've been accepted to present at a certain conference for the last three years in a row. I'm about to submit my work for this year, but beyond it being clear that they'll apparently accept anything after you're accepted the first year, I don't have that much confidence that they'll actually be interested my trash for this year. Maybe it's because I put in significantly less effort and time in than most people so then it feels like it isn't an accomplishment?

Now as to what builds confidence? Apparently nothing in my case. Maybe if I'm ever able to make a comfortable living or if my skills are actually recognized by most of my peers (instead of just a couple) and I'm actually treated like I'm worth something, maybe then I could develop confidence.

I don't know if you relate to any of that at all, but I thought I would share my experience. Especially since I too want to know how to build confidence.