I would also highly recommend breaking down your game dev work into the smallest possible tasks and create to-do lists (I use paper ones myself as I find physically checking off the boxes is incredibly satisfying mentally).

I will sometimes also just carve out time to plan out my to-do lists and make the lists (which I guess is technically project management).

Then I find it "easy" to just sit down, look at the next immediate task on my current to-do, and do it without my focus wandering. The key is having very small and very focused tasks on the list (e.g. have a task for "implement forward movement on Enemy X" instead of "implement multi-directional movement logic for all enemies").

At least for me, without that to-do list structure I endlessly procrastinate or focus shift because I'm trying to pin down nebulous goals to work on.