Well. This could be a lot of different things. What do you think is making you quit?
There is a dopamine hit for an unexplored idea. People essentially get very excited about an idea, like the dopamine, and then when it starts to fade as they have to actually implement things they quit. Sometimes. They manage to get some things down and those small successes of implementing mechanics gives them the dopamine to continue - until they hit a big enough road block. (Guilty!). Working on a game is more about discipline. Not feel good emotions. If you’ve never created a game before create a small project idea, and work on completing it. Even if you don’t feel motivated.
Motivation doesn’t usually last a whole project. But doing it for the sake of doing it will. After you have a small project break it down into bite sized pieces that you can check off from a to-do list (if that works for you). And Don’t burn yourself out on it, but try to enjoy the journey, and keep that finished game goal in mind and look forward to it knowing it just takes overcoming small hurdles repeatedly.
This is harder than it sounds, and maybe it won’t work for you, but it is one way to tackle the problem of completing a project.