It's poetic and all to say "never", but that's not always feasible or realistic. It depends on the story. I realize that's sort of a non-answer, but really, if you've poured all you could into a story and it's not working, it's better to move on to something new rather than to continue to spend your time and energy on something that's draining you. Ask almost any published author, and they will tell you that there were stories that they had to let go of. It happens. There's no shame in it.
When the world you've created is no longer a happy place for you. Because if you don't like the story then you can't connect with it, and then how can you pour everything you have into it?
One does not simply give up on a story. You evolve it. Make it better if you do not like it or end it is a good way in which readers would be satisfied.
Never. You edit and you reconsider but you should never not give a proper ending to a story you started. If you're feeling stuck, find some sort of inspiration. Best of luck to you.
