The brilliant Oliver Burkeman points out the effort of willpower is one that faces us every day, across lots of different situations. Worryingly, it is a finite resource which we can waste on resisting a donut, whilst we fail to veto another slice of jam on toast.
He mentions Baumeister's experiments that showed that exerting self-control uses up real energy. So no wonder we break diets or need food to keep us at our desks.
For once, I think this tells only part of the story.
Willpower is finite depending on how you picture it. If resisting the donut is just one part of a larger, unifying purpose, my bet is that it is much easier to picture oneself also resisting the toast. And if your image includes a new, harder, sportier you, you'll probably then take yourself off for a run and make do with a banana. Purpose unifies goals, so that the drain on resources is lessened. If you can find ways of rewarding yourself in the meantime - this is still important - then your purpose will protect you from being drained.
I will be testing a very similar proposition in my upcoming dissertation which I think is very exciting. More on this later - I'm off for some jam on toast.