I suppose I have a different take on self-discipline.
I see it as the focus necessary to do what you believe is right, in the face of challenge or instincts for otherwise.

The Japanese tea ceremony is something which was/is still learned to the finest micromovement. So much so, it is an art. People choose to control their thoughts, movement, breathing, and actions to complete this simple task. Buddhist monks harden their body and open their minds with self-discipline. Soldiers learn to stay calm and focused in a firefight. Firefighters control their fear to go into a burning building, which is against our nature.

A young child may will themselves to clean their room, when really they just want to go outside and play. Walking by a SALE sign, doing homework, getting up on a Monday morning. These all take self-discipline. It is the set of skills and application thereof, to do what we think must be done.

It is easy to be lazy. Discipline is the opposite. There is a choice. The results of self-discipline are smarter, stronger, better people who will contribute more to society. I personally strive every day to apply and reinforce my own self-discipline. That too is a choice.