Apollo brings up some good points. In terms of will power as in doing what you are supposed to do because you gave your word or it is what is expected that you should do this--yeah, I'd say ISTJ, as they tend to do what is expected or die in the attempt. However, in terms of digging deep within yourself and doing what you think you should do because it makes sense to you and is right for you? (think dieting, quitting smoking, adhering to a specific, self-imposed regimen of behavior) Not an ISTJ trait. I would say while any type can do this, depending on the individual, and any type can fail at this, again depending on the individual, it would have to be ENFP FTW.
An ENFP has something inside of them that allows them to lock into a goal and forsaking everything else, zero in with laser like intensity to achieve that goal. That, coupled with their incredible talent for thinking of outside of the box solutions to problems, these guys and gals are simply unstoppable in making sure what is important to them, happens.