Some people conclude that each person only has a certain amount of willpower to use everyday. It's a great idea that I've believed in and it seems to make sense because you can see it everyday as people "burn-out" doing things they don't want to do, like studying, working, not-eating, etc. But, I'd like to present a different side of this.
Thinking that you only have a certain amount of willpower to spare is just an excuse to give up. So, forget about willpower, it may be the wrong line of thinking if you want to reach [more] goals and give up less.
So, what do you do without willpower leading your life? You live and do whatever you want to do, without thinking/worrying about any "scientific" or psychological reasons/excuses. Whatever your current goal is, focus in on that one. Remind yourself every second, minute, hour, and/or day to get your top priorities done first (see my post on Priority Lists). Caveat: Your well-being typically comes first, which includes sleeping and possibly eating.
Any weakness is just in your head, and you can overcome it.
My original note-to-self said:
Infinite willpower by not knowing what willpower is.