Motivation is usually the reward for starting, not the ticket to get in.

Waiting to feel like it reverses the actual order. The feeling tends to arrive a few minutes into the work, once your attention has narrowed and the task has become concrete rather than imagined. Before that, all you have is the anticipation, which is reliably worse than the thing itself. Starting is what generates the state you were waiting for.

Agree with yourself to begin badly for five minutes with no promise of continuing. Put the first sentence down, do the first set, open the file. Decide whether to stop only after those five minutes, not before.