The rule of thirds says that when you are chasing a dream or doing something hard, you should feel good a third of the time, ok a third of the time, and crappy a third of the time.
Feeling good all the time suggests you are not pushing hard enough, while feeling crappy well beyond a third of the time can signal fatigue or burnout that deserves attention.
(The Rule of Thirds: Credit to Alexi Pappas)