In a creative endeavour, set yourself a goal to stop at. Sometimes that’s a specific time to stop at, but sometimes it’s a specific quantity of work, or a specific point in the project itself. Whatever it is, make sure that you have points you’re determined you’re going to stop working.
Then work right up to that point.
Right now, I have an assignment that’s due tomorrow, at 5pm. It’s done. It’s sitting on my HD, done. I’m not submitting it today, though. I’m going to go over it again and again a few more times, to improve it. I have that opportunity. I have that time. I am going to use it.