Willpower - is it an energy thing? This is a great question. The antonym to ἀκρασία is ἐγκράτεια. Both are based in κράτος, meaning power. One without and one with. I do agree that thumos may be able to provide the strength, but I also think the ancients saw all around them people who were grossly dysfunctional. Just like today. Probably most were dysfunctional. Not everyone had the luxury to "work on their issues." This is why philosophers tended to hang around kings (Seneca/Nero, Aristotle/Alexander, Plato/Dionysius, etc) and why Diogenes laughed at them. He said they were slaves to kings.
So I think if it takes a king's money to be able to reach ἐγκράτεια, I am screwed. I find that self control is so much easier in some certain situations than others. Health, vocation, family support, financial security,etc. How many of us could ever be like Diogenes? Where life could hammer us and we still have the same steel will as before?
So I do believe ἐγκράτεια and ἀκρασία are byproducts of Fate. You may fight to preserve the one over the other for a while, and you may make it to the end! But, depending on where you live, the forces around you, etc etc etc......it seems more and more a crap shoot.