Certainly I don't know how one develops self-confidence when they've never had it. I think back across my entire childhood and can't remember my parents praising/complimenting/saying something nice about me a single time in the entire time (maybe one instance will come to me). That might have something to do with it (I remember the occasional praise from grandparents or teachers). It's not that my parents always criticized me either, but that I can at least recall some incidents (theirs and grandparents, teachers, etc.) and I lived in fear of their judgement.
Add to that they didn't have confidence themselves. And then add to that the occasional remark in anger "having kids worst mistake I ever made!" and ... I really did thinking having kids must have "ruined their lives". Add to that other struggles with peers growing up. So ... I don't know how one has confidence.
But I do know one should try to learn to FAKE confidence (wish that I was better at faking confidence at job interviews, things would be better, got to try to get better at this I guess), learn to act as if they are confidence, learn to perform it well enough to play the part. Because even though it might be ridiculous, we will live in a world that respects raw confidence (more than ability sometimes? well who is President again?!)
I have heard it said that helping others might increase one's confidence.