I think self-doubt is an ongoing, human-wide condition felt by everyone at some point or another. Even the most confident among us has an insecurity or two.
Sometimes self-doubt manifests in the belief that we are a fraud, and that someone is going to expose us. It should serve as a comforting thought that even the top experts in a particular field will experience this feeling.
I think the thing about self-doubt is that you should not to expect that you can eliminate it, but rather that you can learn to manage it, or simply work through it.
There are no certainties, and confronting that vulnerability of not knowing, will give you the space you need to figure it out.
In the end, we are all just making this up as we go along.