Difficulties strengthen the mind, as well as labor does the body.

Mental capacity works on load the way muscle does. Something slightly beyond what you can currently manage is what produces the adaptation; something far beyond it just injures, and something well within it maintains you at exactly where you are. The sizing matters as much as the effort. Most people either avoid load entirely or take on far too much at once and conclude they are not capable.

Find a problem that is a little harder than your last one rather than dramatically harder. Work at it until it stops being difficult, then raise it again. Steady increments outperform heroic attempts by a wide margin.