Cut until it's finishable
The game you ship beats the one you describe.
Not motivation. Scope, and a build nobody outside your machine has ever played.
Ambitious first projects collapse under their own weight. Cutting until the game is almost embarrassingly small is what makes finishing possible, and finishing is the skill worth acquiring first.
Weekly builds keep it real. A playable build every Friday exposes the systems that don't work yet, and makes progress visible when motivation gets thin.
Playtesting supplies the rest. Watching someone else play silently for ten minutes teaches you more about your design than a month of thinking about it will.
The game you ship beats the one you describe.
A playable build is the only honest progress report.
We're not a publisher, and nobody can promise sales.
Twenty minutes, free, on your project and what to cut first.