This is what I've been thinking since I've first read that willpower is depletable: "Isn't it just a self-fulfilling prophecy? Don't I feel out of willpower just because I'm convinced that it should be depleted by now because of all things I've already done today?"

http://www.stanford.edu/~gwalton/home/Publications_files/Job...

Much recent research suggests that willpower—the capacity to exert self-control—is a limited resource that is depleted after exertion. We propose that whether depletion takes place or not depends on a person’s belief about whether willpower is a limited resource.