Willpower is a muscle, according to this from the NYT. You exercise it; it gets stronger.
Willpower and persistence are relatively new concepts for me. Most of my life I've assumed that if something doesn't go well the first or second time I try it, that means it's not going to happen. The idea of regrouping, coming back, regrouping, coming back, etc., still seems like a revelation. These days I'm surrounded by highly successful, highly persistent people. Some are almost like machines, moving inexorably forward--they roll into a closed door, bounce off it, and roll at it again until they push it open.
I also learned about persistence from Norman Fischer's book Taking Our Places (and thanks to Sara for this recommendation). Zen meditation is the practice of persistence, especially because, in my experience anyway, it is not immediately rewarding. One sits there and wonders why one is bothering. Enlightenment does not come; relaxation, even, does not always come. Yet one sits down and does it, at the appointed time, again and again. (I say "one" because at the moment *I* am not doing it persistently, or even very often. OK, but I did get the concept; really I did. And I am persisting in my writing, despite hitting a dismaying patch that I won't go into just now.)
A big part of persistence is recognizing the value of what you're persisting at, even--or especially--if no one else does.
Stubbornness and passive or active aggression are not persistence. It can't be done in anger or revenge, or to prove a point. It should not be destructive. This is why I'm having a hard time admiring what Hillary Clinton calls her "resilience." Yes, she's a survivor, but that seems to be all she is and all she can ever be at this point. It's not her fault, entirely--she's had to make compromises and those compromises have trapped her. But surviving is not governing, and I'm afraid it's not even a good example in this case.