Well, that was the conclusion of a group of researchers in an article in The Journal of Personality and Social Psychology (1998). You can read about it in the new book Switch: How to Change Things When Change is Hard, by Chip and Dan Heath (2010).
The researchers demonstrated that when people are put in a situation where they must exercise self-control (by not eating chocolate chip cookies when they are mildly hungry), later in the day when they are required to concentrate on a hard but solvable problem, the subjects who had to resist eating the chocolate chip cookies gave up twice as quickly (8 minutes vs. 19 minutes) and after barely more than half as many attempts (19 vs. 34 attempts) as those subjects who had been allowed to eat the cookies! Apparently, they ran out of self-control.
The researchers’ conclusion: Self-Control is an exhaustible resource.
So if we want to stop or change any habitual behavior we need to realize the No. 1 tool we’ve been taught from childhood to rely on, self-control, will not get us where we want to go. We don’t have enough of it. No one does.
Instead, we need to factor in this limitation and use our self-control carefully and wisely to create and establish new, automated behaviors that take us away from where we do not will to go and in directions we do want to go.
For more information on the differences between will and want, see Lesson Two of The Quick-Start E-Course to The One Minute Cure for Porn Addiction.