What kills your momentum after the prototype finally works?

The first playable build feels like a breakthrough. Then the work changes: edge cases multiply, every “small” change risks breaking something else, and progress becomes harder to see.

For people who have shipped—or abandoned—a game or software project: what caused the momentum drop after the prototype?

Was it scope creep, testing, content production, platform bugs, or simply losing confidence that new changes would not break old behavior?

And if you could automate one part of that post-prototype grind, what would it be?