Motivation is a passenger, not a driver
We are taught to wait for motivation and then act. That order is backwards for anything that matters. Motivation is a mood: it shows up unreliably, usually for things that are already easy, and it disappears exactly when the work gets hard.
If your system requires you to feel like it first, your progress is hostage to your mood. The people who look disciplined are mostly people who stopped consulting how they feel before starting.
Action generates the feeling
In practice the sequence is: start badly, feel a small amount of progress, and only then feel motivated. Two minutes of movement produce more motivation than an hour of waiting for it. This is why the hardest part of any session is the first ninety seconds, and why nothing you do before starting really solves it.
Momentum is just this effect, compounded. Each small completed action makes the next one slightly easier, until continuing is genuinely less effort than stopping.
Lower the bar until starting is trivial
When you cannot start, the task is too big in your head. Shrink it to something almost absurd: open the file, write one sentence, set a timer for two minutes. You are not committing to finish, only to begin.
A visible countdown helps enormously here, because it converts an open-ended dread into a bounded, finite thing. You are not doing the whole project, you are doing twenty-five minutes.
Protect momentum more than intensity
A short session today is worth more than a heroic session next week, because momentum has a half-life. Long gaps make restarting expensive, so consistency beats intensity almost every time.
On the days you have nothing, do the minimum version anyway. It keeps the chain alive, and the chain is the thing that carries you when motivation is nowhere to be found.