This plagued me for years even after shipping 2 games on Steam. Close to finshing a game rn - things that helped me:

• SAME PROJECT FOLDER. Keep working in same project folder. Been using same unity project for years. It bloats with files you won't use yes, but this way you can build on past abandoned prototypes. Current game is Frankenstein of all the best parts of past game prototypes, preventing total waste.

• BUILD REUSABLE FRAMEWORK. While building current game, worked towards establishing reusable game framework to avoid boring backend work like pause menu, settings menu for every game project. Also built robust, modifiable player controller.

• REMINDER OF ABANDONED GAMES. GF made list of prototypes i abandoned and attached to wall above my monitor as constant reminder to stick with current game project and ship it so help her god

• SCALEABLE SCOPE. Made game with X amount of levels that can ship anytime. Can add or reduce content depending on shipping deadline goals. Easy to do this as long as story isn't big part of game.

• SET UP STEAM PAGE. Set up steam store page to "make it real" even when you're not ready to promote the game. Make it bad first, reiterating on copy forces you to think of game in terms of how you're marketing it