Notice that reward makes it easier over time, not discipline. It hooks you to the activity making you no longer complain about lack of motivation. If you fail to develop such receiving of reward after the activity, then you would find it hard to continue, blaming it on either motivation or discipline. If you need discipline to continue, then chances are, you are gaining nothing that you really want from the hard thing. This is aside from it being supposed to become a habit. Still, discipline makes you able to do something you hate at first only – not later on – until you arrive at the stages of it becoming easier because of it becoming a habit, and you feeling more motivated toward it not by pep-talk, but because of the reward you expect to receive at the end of the activity each time you do it from now on.

If you need discipline to keep doing a certain thing over and over again, chances are, you shouldn’t either be doing it, because this means you hate it, or your problem is more general than just hating this certain thing.