i wouldnt say being unable to finish a story is entirely about willpower. its part of it. but not all of it.
if youve never written a story beginning to end, it can feel like a mysterious process, and its a surprisingly big hurdle to get over.
OP, id suggest you set yourself a challenge. give yourself a month to write a complete story - lets say 10k words, keep it short. maybe do a week and go for 1k words - meet yourself where youre at, and if you fail, lower the bar. but once this month / week / set period of time is done, you are not allowed to touch it, and must post it.
that way, you get a practice of just completing things, marking them finished, and moving on to something new.
also - plan! plan a lot! if you start a story by sitting down and writing 'once upon a time...' you will struggle, if you start working with:
' - introduce steve
- steve needs to get to the shop
- the roads are flooded
- steve cant swim
- CONFLICT!'
(... or smth like that) if you start with notes and go over the entire plot like that, youre focusing on the bones of your story and not getting bogged down in the details and flourishes of the action and prose - thats the muscle and skin, it comes later. at this phase you can brainstorm, you can go in one direction and then change your mind, and once youre done you will know, for certain, where you are going. being able to see the end makes it so much more likely that you will get there.
and once youve done this once, doing it again will get easier... and easier... until youre ready to take up that big passion project you have in your head. first, do some stuff youre not expecting too much from, just to flex your muscles, and practice sticking to deadlines. and yeah, thisll include willpower, but if you plan properly youll have tracks to your train of thought, and pushing forward will be so much easier.
also
yeah. when you can, turn off the computer and work physical - esp for planning. less distracting, more intuitive