I've been a carpenter since I was 18 and a finish carpenter since i was 30. I'll be 52 this year. So I've been doing this for 34 years. By all regards, I'm an expert in my field. My work has been on magazine covers, my work has won awards for architects and designers. I'm known by name by top builders and firms in my area. I now run jobs as a superintendent and/or project manager. I get calls to come work for other companies on the regular.
I still have zero idea why. Like, I just tell people what to do in an order that makes sense. And before that, I beat nails into wood. It wasn't till about 3 or 4 years ago when my wife took me aside and explained to my face that, yeah, I'm really good at my job that I started to realize that, yeah, I'm pretty good at my job. But everyday, driving in, I'm still just a kid that's in over his head. I don't feel confident at work. I know on one level I'm doing OK, I mean I must be, right? But I just come in and do my best and hope it's good enough. Turns out, that it usualy is good enough.
And I can tell you this. Anyone that walks around super confident in their work, usually sucks at their job. I've seen dozens of people claim that they're the best around, only to get axed or laid off as soon as possible. Don't bother being confident in your work, be confident that you're doing the best that you can do and be confident that you have the ability to keep learning.