If by self discipline you mean sticking to a commitment or a routine than you need to learn the science of habit formation.
In his book “The power of the habit: why we do what we do in life and business”, Charles Duhigg explains it brilliantly.
There are three steps that forms habit. Every habit, good or bad consists of these three steps.
Cue: This is a trigger for the habit. Take smoking for instance. For smokers the cue could be the smell or the itch they feel when they want a cigarette.
Routine: Routine is the habit that you follow through on after the cue. It is the act of performing the habit.
Reward: It is the pleasure that you get after performing the habit. It releases dopamine in the brain which makes you perform this habit over and over again.
In order to break a bad habit you need to change the routine of the habit but you need to keep the same reward. So if you feel the need to smoke, you go for a run or do ten push ups. This will ensure that you get some reward at the end of your routine. You have to do this for 21 days to wire your brain for this new habit.
In order to form a new habit, you need to set up a cue and a reward for your habit. If you need to start the habit of going to the gym you have to set up cue such as a particular time of the day or an alarm on your phone. The reward is going to be the chemicals that will be released by your brain such as endorphin and dopamine that will make you feel good.
Self discipline is all about forming good habits and removing bad ones.