There is always capacity to choose not to act on a reactive impulse. The notion that that capacity is a finite resource which can be depleted is an illusion.
Resisting reactive impulse is one thing, but do you not find that you have a limit when it comes to willing yourself to do difficult things? Assuming that these things come one after another, of course.
I'm willing to agree that capacity for hardship is infinite in a sense, but I would also contend that it gets harder and harder to call on your willpower/self-control if you've already had to do so many times in the same day.