If i were to define diligence i would say that it is means being tough and candid in order to prevent harm. Very often we let small or big things slip because they don’t matter very much for and by themselves, but accumulated and as a part of a whole they often are disastrous.
Being diligent is a style of living your life since it permeates everything. It is very different from being disciplined which is more about sticking to a predefined plan. It is about being diligent in everything and each moment of your life.