Discipline is something we develop over time. it begins with small accomplishments and the forming of small habits - making our bed; etc. The development of discipline comes from making small promises (to ourselves) and keeping them - which is ultimately, about ‘integrity’. It also arises from our ‘character’ - and traits like grit, fortitude, resilience all come into the mix that coalesces in to the thing we call ‘discipline’. And it is all these things, that see us through, when things turn bad, when times are tough, when motivation is low.
When motivation is high - we power forward toward our goals - when motivation is low - character, discipline and habits are what keep us going. When seas are smooth and the wind is perfect, motivation sees us sailing smoothly - when the storm comes, discipline and character will get the job done.