I was and remain skeptical of the notion that will power (self-regulation, putting-off-the-marshmallow, whatever you want to call it) is trainable, and I still find it difficult to imagine what the mechanism of that "training" might be, but I don't know how else to explain the steady progression in my self-control. There were changes in my life a year and a half ago that substantially reduced my stress, and which made bringing my eating under control possible, but those were one-offs, and this development seems to keep going. There's all kinds of ways in which I can self-regulate now, all kinds of planning I can undertake and carry out. Eating, spending, exercise, social media fritter: they're all becoming workable.
I suppose an explanation that doesn't involve the will being generally trainable would simply be that when you've got more will available, you construct new daily routines, and as those daily routines become habitual, the will you used to construct them becomes available for new construction. That would account for the appearance of steady progression. It looks like more and more self-regulatory power, but actually it's just that the steady finite surplus "accumulates" in the form of habits.
There also of course is just an enormously increased sense of self-efficacy. But none of this quite explains how easy it now feels -- how progressively easier it feels -- when I encounter a temptation, to shrug my shoulders and say, "sounds nice, sure, but I'm not someone who yields to temptation any more. That's not who I am."
On the other hand, I am in a long, slow, obscure developing crisis: the grandiosity of my former self -- the flip side of yielding to temptation -- has also been going away, and it leaves me up against my limitations and my mortality, in a particularly bleak way. In the last third of my life (assuming luck), what can I do that's of any use to anyone? How can I serve? I don't know. So many of my basic assumptions are dissolving, in this corrosive political atmosphere. I need to find or make a compass: I need to wake with a purpose. Eating right and exercising are well and good, but they're instrumental goals, not ultimate ones. The point of maintaining a car is that you eventually mean to drive it somewhere.