Learn English – What’s a word for “desire to improve theself”

Iterate is the word that's generally used for this, I believe:

The program iterates through the collection, searching for every occurrence of the target value.

Your "willpower" really does sound like the most literal definition of "mental endurance", but based on your sample, there are a few ways to go with this.

First is to focus not the abstract concept of mental strength over time, and instead on its results:

...but has issues in terms of confidence and consistency

or

...but has issues in terms of confidence and self-discipline

or just "discipline"... but this might have the connotation that the person as trouble with authority.

If you just mean the person's ability to doggedly stick to it:

...but has issues in terms of confidence and tenacity