Discipline is often described as force. At its best, discipline is remembrance.
It is remembering the life you said mattered when comfort offers a cheaper decision.
Motivation depends on emotional cooperation. Discipline allows the principle to remain active when the emotion changes.
A disciplined person is not someone who never drifts. It is someone who notices the drift and returns before it becomes identity.
Every kept promise tells the mind: I can trust the person making my decisions.