Isn't this more like a fight between instinct and rationality?
Wanting the pie is rarely a rational decision, while on the other hand wanting to be healthy so you can live longer is absolutely rational.
Hence willpower is one's ability to rationally control their own anamalistic instincts and emotions.
Sure wanting to eat the whole pie might be irrational, but that's seldom the way people think about it. I can't eat a whole pie at once, but I can definitely eat the bite on my fork, add one more slice to my plate, etc.
This mistakes thinking long-term vs thinking marginally, which is the larger point being made.
I could certainly be convinced otherwise but it’s interesting to think about.
If you do it out of instinct and immediately think "oh yeah, forgot I only need to look one way" then it wasn't logical.
But if you know it's one way, yet think "I've witnessed bikes and even cars going the wrong way before, better check both directions" that seems like it could be a perfectly logical action.
A reason alone doesn't make something rational. A paranoid thought that aliens are trying to abduct you may be backed up by the reason "because secret weather balloons are tacking me". It's a reason. It's not a rational one. Having a reason is not synonymous with being rational.
But those are the exceptions. Mostly it's the other way around, as you say. People undermine their own long-term interests with procrastination or impetuousness.
is not necessarily
> wanting to be healthy so you can live longer
the behavior necessary to lose the weight would have positive results toward a goal that IS rational, but the impulse that actually spurs the behavior necessary to be two sizes smaller by next summer could just as easily be "i want to wear this dress to my sister's wedding next summer and they don't make it in my size."