The strongest belief in yourself includes room for improvement.

Believing in yourself and acknowledging that you have a long way to go are not in tension. The strongest version of self-belief is confidence in your capacity to grow, which requires accepting that you are currently incomplete. Belief that depends on already being finished is very easy to break.

Write down something you are genuinely good at and something in the same area you are still bad at. Hold both without flinching. That combination is what allows steady improvement without constant self-doubt.