Become harder to stop than you are easy to discourage.

There is a difference between how easily you get knocked down and how quickly you stop. Discouragement is a feeling and it will keep arriving whatever you do. Stopping is a decision, and it is the only part of the sequence you actually control. When the gap between those two widens, disappointment becomes weather rather than a verdict, and you keep moving through it.

Notice what you normally do in the hour after something goes badly. That hour is where the quitting usually happens. Decide in advance on one small action you will take in that hour, something so modest that a discouraged version of you can still do it.