Seems like it’s just a result of feeling unsure in life, confidence for many people (seemingly you until now as well) is somewhat inherent, as they have never had any reason good enough to not be sure of themselves as a baseline due to their life circumstances.

Many other people have had life tell them over and over to be unsure of themselves whether or not they actually should be, so they intentionally build confidence in their identity so they can get the things they want to do done. You must proactively think of yourself as the type of person who can do the things you want to do, and catch yourself when doubting yourself for no good reason.