Knowledge has to be improved, challenged, and increased constantly, or it vanishes. Peter Drucker
Just like knowledge, you can’t take self-discipline for granted. Unfortunately, being a self-disciplined person isn’t a “one and done” kind of thing. Once you have learned how to live that way, you can still lose it if you don’t consistently strengthen it by setting new challenges and rejecting instant gratification in favor of bigger future rewards.
Never assume that you’re “disciplined enough.” There’s always a new area in which you can improve your self-control and further expand your comfort zone.
For example, regular exercise poses no challenge for my self-discipline. In order to strengthen it, I need to set bigger and bigger exercise-related challenges for myself.
Instead of focusing on fitness, I can also find a new area in which my discipline is lacking (such as developing more patience when dealing with other people) and focus on improving it until it’s no longer a test for my resolve.
Such consistent practice ensures that you’re at least maintaining your level of self-control, and ideally always getting better at it.