The Empiricist

Apparently there’s a limit to willpower. Force yourself to do enough things that you don’t find easy or enjoyable, and you run out of willpower to keep making yourself do new things.
http://www.futurepundit.com/archives/006569.html
This is why it’s a good idea to make changes to your life one at a time. Routine is the solution here. Once something becomes routine, you no longer need willpower.