You have a finite amount of willpower that becomes depleted as you use it. Your will, in other words, is not a manifestation of your character that you can deploy without limit; it's instead like a muscle that tires. This is why the subjects in the Hofmann and Baumeister study had such a hard time fighting desires--over time these distractions drained their finite pool of willpower until they could no longer resist. The same will happen to you, regardless of your intentions--unless, that is, you're smart about your habits.