Motivation runs out. Curiosity refills itself every time you feed it.

Motivation is a supply you spend, so it works well at the start of something and badly in the middle where most of the work lives. Curiosity behaves differently, because every answer opens two more questions, so using it makes more of it. People who are genuinely interested keep going without being pushed. They are not more disciplined, they are running on something that refills.

When the drive fades, stop trying to summon it and go find the interesting part instead. Ask what you still do not understand here. One real question does more for a stalled week than any amount of psyching yourself up.