Will Power - A Resource of Diminishing Returns.
So you're not a well trained dog, but don't beat yourself up. If you can't resist your favorite treat it may be because you have used up your willpower quotient for the day. Both experience and experiments suggest that will power is a resource of diminishing returns.
Sometimes it's just a matter of having too much on our plates. Kelly McGonical, author of The Willpower Instinct says deciding what to eat on a daily basis requires 227 discrete choices. With so much complexity, fatigue sets in and the brain loses focus.
There have been several famous studies about will power and self control that use food. In 1970, the bait was marshmallows. In 1998, cookies were supreme. In an experiment constructed by psychologist Roy Baumeister , participants were asked to choose radishes over cookies and then solve a complex puzzle. The radish eaters spent significantly less time trying to solve the puzzle than the cookie eaters. One conclusion to this event was that the poor performers had "run out of self control." Furthermore, it developed that self-control is a general strength that's used across different sorts of tasks -- and it could be depleted.
Self control became analogous to a muscle that could be improved by regular exercise or boosted with glucose. Professor McGonical states that when glucose levels rise we can defer self-indulgence. So if thinking doesn't work, perhaps we should drink a glass of orange juice when confronted with a difficult choice.
Cognitive dissonance suggests we are not rational beings and justify behaviors that are in opposition to our views. The desire for a cookie and being on a diet is a classic example. We know we can't have a cookie and still be on a diet, but we eat one anyway. And then we eat them all because the diet has been blown. Again, the correct choice would have been orange juice.
Since self control, particularly around food is difficult, the place to make the hard choices is in the grocery store. Common sense suggests that you shop after imbibing orange juice,or after a meal, never when you are hungry. Confine yourself to the perimeter of the store. Do not buy forbidden foods. Now, when you are too tired to exercise self-control, you won't have to because those decisions that fatigue willpower have already been made.