Very important reply to the question, "Is willpower depletable?" by Kat Li, who has an MA in psychology and gives a well-researched answer on Quora:

"Willpower is depletable only if you believe it to be so, is the message from Stanford psychology professors Carol Dweck and Greg Walton [1]. Though past research has shown that willpower is limited and dependent on a continuous stream of glucose, the story is more complex than that. 

Dweck, known for her groundbreaking research into the world of mindsets and achievement teamed with Walton, an expert in theory-based interventions to devise a set of experiments looking into willpower [2]. They found that beliefs about whether willpower is a limited resource affect performance on difficult tasks.

One of their studies examined beliefs that subjects held about ego depletion by asking them to rate how much they agreed with statements about it (i.e., "After a strenuous mental activity your energy is depleted and you must rest to get it refueled again.") Then, the subjects completed either easy, almost mindless tasks or a more complicated ask involving self-control. Following that, both groups completed a Stroop task, which is a standard measure of ego depletion. 

They found that subjects who believed that their energy could be depleted did perform worse on the Stroop task after having completed the more cognitively taxing task. However, participants who did not believe that energy was limited performed no differently in the Stroop task, regardless of whether they had done the easy task or the challenging task.

Moreover, in another study, they had subjects read statements such as, "Sometimes, working on a strenuous mental task can make you feel energized for further challenging activities," which caused all subjects to look like the group in the first study who believed that willpower was not limited. Compared to participants who read statements about the limits of willpower, these subjects made half as many mistakes on the difficult task."

So -- stop thinking of willpower as depletable? In fact, it's just the opposite -- after you do something difficult, you have more energy. Remember that and make it a mantra. After you do something difficult, you have more energy.