Rhetorical question: How do you know when to set a ms aside? How do you know when to push through? How do you know when to abandon it entirely?

I have 2 mss I can think of that I set aside forever. One was never going to work; it was mostly for my own enjoyment. There wasn't a theme or any kind of depth to it. This was early on in my writing career, and I came to see that it was never going to amount to anything worth showing to anyone else. But I loved writing it so much that I did a good portion of it by hand in a notebook because my computer died and I had nothing else to write on. Tens of thousands of words, by hand--I wouldn't do that nowadays.

I started another ms--it may have actually been an offshoot of the above ms, now that I think about it--and saw very quickly that it could not work because the MC was lower midgrade and the storyline and voice were YA. This might have worked if it had been an adult book, because sometimes those do the older-voice-looking-back. But it was a YA storyline, and not much of one at that.

I have started and set aside w-f-h ideas, but those don't count.

I still have the first ms I ever completed, and still intend to figure out how to make it work. Why? I don't know. I just feel there's enough to it. I feel it would be wrong to let it languish and die, because it is still viable, so to speak.

I kept at Night Road long past its sell-by date. Fourteen years after starting it, I figured it out and made it work.

That's very big picture thinking: Will this ms make a book or not? But what about smaller issues:

Should I keep working on this WIP or set it aside to work on something else?

Am I making this ms worse by pushing and gnawing at it, and would it be better served if I put it down and stopped thinking about it?

Those are the questions I don't have answers to, and don't know how to even start to answer. I wonder if most other writers know the answers. I wonder if everybody has a different answer and has to find it out for him/herself. I wonder if every single ms has a different answer, even mss by the same writer.

I wish I knew more about what I was doing. I wish I could be certain of a minimum amount of progress in relation to the amount of work I put in.

I do know that when I made the decision to set those unworkable mss aside, I did not feel bad or regretful or even frustrated. It was just like, well, this is never going to work so let's move on. No relief, no nothing. It was like sweeping a dead bug out the door.