“Self-respect is the fruit of discipline; the sense of dignity grows with the ability to say no to oneself.” ~ Abraham Joshua Heschel; Polish-American rabbi and philosopher.
Plainly put, self-discipline is the ability to make yourself do what you should do, when you should do it, whether you feel like it or not. And as Lao Tzu wisely said, “Conquering others is mere strength, conquering yourself is true power.”
Most people do not have guidance in life, they lack structure and they are always set adrift in expectation for the next moment, and this happens because they are like ships without a sure route, without a destination mapped, they bounce here and there and when things go wrong they complain and feel like victims.
There is nothing more reassuring and excitingly peaceful, where your mind is set on a definite course and does not deviate, like self-discipline because it is exactly self-respect, it is loving yourself so much that you only do the things which are best for your life, the things that keep you in harmonious track–i.e., eating healthy and the right amount, getting right sleep, working when you have to work and enjoying yourself when you have time for yourself, thinking the right thoughts and sharing in good conversation which elevates, etc. etc.
That is called conquering yourself, ruling yourself, which makes you truly a master of yourself and of your destiny.
Self-discipline gives you strength and confidence, and it helps you appreciate yourself enough that you want to better yourself every single day, while becoming more aware of the things you must do in order to be successful in life.