I find willpower self-perpetuating. The more definitive effort and energy I put into something, the more that feeds back into a drive to keep going. Willpower is limited only if its thought of as some divine quality the comes and goes as it pleases. It all just goes back to the old 'starting is hardest part'. A prime example is the essay I'm writing at the moment. I spent a good 5 or so hours, give or take, finding research that supported my thesis. Then it came to finding sources that opposed it. I spent maybe 4 hours without finding anything useful to me. If I hadn't already spent 5 hours good work on it, I most likely would've given up and thought of different idea. But because I had already put in that effort, it was easier, or perhaps I was more naturally inclined, to maintain that drive. It paid off too because I found all the stuff I needed. My Google-fu skills need sharpening I think.