Motivation is a feeling. Discipline is a decision. You do not control the feeling, you control the decision.
Alan Stein's distinction, run on a real team: motivation is a feeling, discipline is a decision with an action. You do not control how you feel, you control how you respond. Rely on motivation and your effort ebbs and flows with it. The disciplined choice is the one that aligns with the person you are trying to become.
The work that actually builds a business gets done on the days nobody feels like it. Waiting to feel motivated is how things stall. Deciding in advance who you are and acting like it is how they get built.