Thinking about... the incredible strength of habit. Breaking a habit is one of the hardest things that people still manage to pull off.
Willpower is finite; you expend it on thoughts and behaviors, and replenish it with rest.
When a behavior becomes a habit, it takes much less willpower to do it. Habitual behavior has inertia, requiring more willpower to stop doing it.
There are skills - learnable actions and models - which are useful in changing habits. I think the name for these skills is "discipline."