The body remembers routines your motivation has already forgotten.

Motivation is an unreliable narrator and a poor record keeper. The body is different. It adapts to what it has been doing, gets used to a certain time of day, a certain rhythm, and it will pull you toward the routine even on days when the enthusiasm is entirely absent. That is why establishing a pattern is worth more than any burst of resolve.

Hold the routine for long enough that it becomes physical rather than mental. Same time, same place, low bar. Once the body remembers it, motivation stops being the deciding factor.