How do you stay motivated for the entirety of your project?

When starting a new project I find myself full of energy and motivation. I can spend days working on it non-stop.
However, once I get to a certain point where the project is taking shape, I find it harder to dedicate time and stay focused/motivated.
Basically when I get to the polishing, modifying, or adding features portion of the journey; I simply start losing interest. How do you prevent this or push through this plateau?
Any & all advice are welcome!