From the beginning of time, diet companies have made us feel like our willpower is what will allow us to succeed. If we fail, it is not the diets fault – it's our willpower. If we just concentrate harder, stop cheating and don't allow for derailments, then we can easily stick to the plan and “suck it up” for the long haul.
Willpower is an exhaustible resource. You only have so much of it and when it's gone, it won't matter what you had planned to do; it's gone. Diets unfortunately, take up a vast amount of willpower. So when you diet, your willpower is drained by trying to stay on track. But we don't get to pick and choose where our self control comes from.
We spend our whole day just trying to keep our willpower in check. Our boss gets on our back, and we keep a tight lipped, respectful reply. Traffic jam on the way home, the frustration keeps rising. We get home and try not to yell at our children for no reason. It all requires willpower, and it's just a matter of time before we drop a ball somewhere.
Dieting in general causes an insane amount of stress on us. Diets tend to be very restricting, eliminating too many food choices, or calorie levels. And dieting during high stress situations is never a wise choice. (let alone a restrictive diet at any time!) When it comes to sticking to any kind of diet, picking one with a dozen different changes to your food intake, will surely cause a catastrophic failure to your willpower. So choose a diet that will change only ONE thing. This will allow you to concentrate on that one change until it becomes a habit.
When something becomes a habit, it will not require your willpower to keep going. So pick the diet that you can incorporate into your life for the long term. Think not weeks or even months, think for life. If you cannot sustain your diet plan for the rest of your days and be happy living your life, then your willpower will run out.