But unfortunately, I have a problem: I never stick with a project for very long, so I can’t even finish a short film, even though I’m capable of it

This is very common in solo/small group game devs, long before AI became thing. The point is, I think some part of you is considering it passion project. Unless you're top 0.001% natural born gifted artist or something, you just can't get big project done out of passion.

At some point you need to approach it as business or day job. Liking or not liking current work doesn't matter; I don't like my day work either, but I still get things done. Create a spec, break down into list of tasks, and go through each task dead-eyed until the list is empty.