Capacity grows when difficulty arrives in amounts you can adapt to.

Growth comes from difficulty you can actually absorb. Too small and there is nothing to adapt to, too large and you break rather than build, and the recovery costs you more than the attempt gained. The whole skill of developing yourself lies in choosing the size of the challenge, which is a decision most people leave to chance.

Before taking something on, ask whether it stretches you or overwhelms you. If it overwhelms, find a smaller version of the same challenge. There is almost always one, and it produces more growth than the heroic attempt that ends in retreat.