Believe in yourself enough to test yourself honestly.

Belief in yourself is only worth having if it can survive contact with evidence. Honest testing, where you might actually fail, is what turns a hopeful self-image into a reliable one. Avoiding tests protects the image at the cost of ever knowing whether it is true.

Pick something you assume you are good at and test it in a way that could genuinely go badly. Look at the result plainly. Whatever you learn, you end up with a more accurate picture and a self-belief that no longer needs protecting.