Below are some prompts and exercises to help you when you need constraints.
Review
What feels too open-ended right now? Where are you facing so many possibilities that choosing one feels impossible?
Where are you making the project bigger than it needs to be? What have you added that isn't actually necessary for the thing to work?
What decisions are you currently trying to make all at once? Which ones could you postpone until you have more information?
What would you make if you had half the time, money, space, or resources? What survives when the extras disappear?
What constraint would actually help you? A deadline? A budget? A format? A location? A number of people? A limited set of tools? Something else?
Practice
Parking Lot: draw out parking spaces for all of your ideas and leave them there. Create a dedicated place for every idea that isn't the thing you're working on right now. Whenever a new idea appears, write it down and put it there. Give your brain permission to stop holding it.
Take the Tiny Version: Take the biggest version of your idea and make it 10% as big. Same fundamental idea, dramatically reduced scope. What would that version look like?
Rule of Three: you can only have three active priorities at a time. Everything else has to go into a box/folder called ‘Later’. When one is finished, something new can come in.