We may know what the right thing to do is, and yet not do it when the time comes.
Elbert Hubbard, the American writer and philosopher who lived in the 1890s had this to say:
Self discipline is the ability to make yourself do what you should do and when you should do it, whether you feel like it or not.
Truer yet harder-to-implement words were perhaps never spoken.
Elbert goes on to say:
There are 999 other success principles I have found in my reading and experience, but without self-discipline, none of them work. And with self-discipline, all of them work!