It’s a problem all hobbyist face sooner or later. It might be a days, weeks, months, or even a few years on a project, but sooner or later the new idea fades and other shiny untested ideas shine.

As far as I know, everyone struggles with it to some degree, and it seems there is no remedy to “skip” it. There are strategies to help, and things that can make success more likely, but you can’t skip overcoming this struggle:

Designing games is quicker and more fun than making games.

Don’t get me wrong, there is tons of fun to be had in making games, but you will have to push through hard problems. You will have to times where you tread water for weeks at a time trying to figure out how to navigate to a solution. And the shiny new ideas will always be there tempting.

My best advice is to start incredibly small. A simple game that you can finish in a week. Maybe it has one level, maybe it has a few. Maybe it’s part of a game jam. But actually finish it. Resist the urge to try something else, resist the urge to quit. It’s going to be a bit of trash that in later years you’ll want to bury, but that’s okay!

Now make a totally different game that’s two weeks long, but as you do, recognize when you are reusing strategies or when the work on your first project could be reused. And when you do, start storing that code up in reusable modules you can just attach to give behaviors to things. In this way, not only will you be forced to learn new things, because it’s a new game concept with new challenges, but you’ve also started to build a library in your mind of how to solve various problems and a library of behaviors you can use to skip forward in the development process. This helps speed up the pace of going from idea to prototype.

Do this a few times. At some point, you graduate from building micro games to building prototypes. When you have a flash of inspiration on a game, spend a week or two making a prototype. It won’t be finished, but it will let you try out that idea and find the ones that are fun.

And then you graduate to vertical slice. Spend a few weeks to three months making a polished vertical slice, something you would actually be proud of, even if it is still no where close to a complete game.

Now go back and look at your first few little games, and see how far you’ve come. If that doesn’t give you the inspiration to see what you can achieve, I don’t know what will.

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.

I can’t finish anything without an external deadline. Otherwise i just keep refactoring and tweaking forever.

Game jams, a self-imposed deadline, promising other folks a demo on a certain day…

Sometimes i pick something as a reward, like playing some game (lately, TOTK) only after having delivered a new feature for whatever game idea.

Another thing that helps is setting smaller goals - a basic first version as 1.0, then small features to bump to 1.1, 1.2, etc. Bigger features can go in later versions! Don’t turn yourself off trying to do it all at once.

I think game dev is prototyping lots of ideas - then at some point you look back and decide one of them is worth a more serious time investment. In the mean time, keep prototyping!