If you want to become a better runner, you don’t work on believing you run quickly. No, you exercise your leg muscles, learn how to use them more efficiently, and practice.
Same with magick. The key isn’t believing you’re good. The key is exercising the parts of your mind that drive magick, learning to use them efficiently, and practicing. I call them mental muscles because, like your leg muscles, they’re what makes the difference between imagining doing something vs actually doing it.
Belief can communicate your intent to your mental muscles. It can set them to task. But once you’ve told them to run, the thing that matters is how strong they are, and how efficiently they move. Once they’re running, believing harder won’t make them go any faster.