I've been reading The Grieving Brain, so my thoughts are more focused than usual on the lens of grief. But we were talking in one of my creator groups about that point where you the creator think everything is terrible and really this making things was a bad plan and someone should have stopped you phase. I referenced an old Gaiman post about that: https://ywp.nanowrimo.org/pages/peptalk-gaiman
And similarly Yarn Harlot had a post about the feeling of coming to the end of a yarn project.
And I think in both cases, it's grief. Maybe not explicitly or exclusively. But part of what The Grieving Brain talks about is that part of the work of grief is your brain adjusting to the difference between what was and what is.
And that happens when you make things. The difference isn't necessarily bad, but in the execution things shift and change.
I do also think some of it is anticipatory decision making. Like once I'm finished what next? And even if the answer is editing or blocking or finding the next project, it requires shifting gears.
And so while you can trash it. Or stop. Or go write something else. You can also keep going. Partly because it is easier to evaluate a finished project. And this is not to say finish everything no matter what. If the sweater clearly will not fit, then stop.
But also, some of this feeling bad is an expected part of the process as you realign your expectations with reality. The feelings are still there, and still valid. They just don't always mean stop.