People say, "If you don't believe in yourself, why would anyone else?" And the intention behind that is good, and it can be helpful.

But here's another angle, because why not?

That question is a kind of mind trick or crutch. It makes an excuse for believing in yourself: "Oh, I doubt myself, and maybe I really am not capable at this thing, and untalented, and will never make it, and that could be predicted from what I already know. But when no one believes in you, you're still justified in believing in yourself - because it's logic: nobody believes in you unless you believe in yourself, so believing in yourself when it seems like no one in their right mind would is a prerequisite for anyone believing in you in the first place. Therefore let's put these doubts aside and lay the foundation of any and all belief in myself."

It's kind of a subtle thing, and I don't think most people parse through the implications quite like that. But... I suspect that's a lot of what's going on in the frame of mind evoked by that rhetorical question.

Anyway, what I'm saying is: you don't actually need an excuse. Believe in yourself because you exist, and by existing, help to create the universe.