I think that Will is developed and honed by Practice—just like many other things.
When you feel better it's easier to exercise it.
Also, when you have a goal firmly in mind.
Once you establish good Habits, less Willpower is called into play.
That being said, People Fall into Error and Bad Ways.
You never hear of someone Falling into Truth or Better Ways.
Improvement is—almost by definition—an "Upstream" type activity.