I think there is definitely something to be said for "executive control" being limited, those networks get "fatigued" like a muscle, etc.
Willpower is tricky because as soon as you start to build a habit, you change your expectations and emotions around it. You can even change those before taking action, with things like visualization. Once something becomes enjoyable/desirable/associated with positive affect, it's a lot easier to continue with even if it requires significant "anti-habit" effort.
Perhaps it's really "effort in the face of negative emotions" that is limited.
Even then, it's probably trainable and extendable with exercises that improve executive control, e.g. dual-n-back, etc.
Willpower is tricky because as soon as you start to build a habit, you change your expectations and emotions around it. You can even change those before taking action, with things like visualization. Once something becomes enjoyable/desirable/associated with positive affect, it's a lot easier to continue with even if it requires significant "anti-habit" effort.
Perhaps it's really "effort in the face of negative emotions" that is limited.
Even then, it's probably trainable and extendable with exercises that improve executive control, e.g. dual-n-back, etc.