I let go when the save file just can't go on anymore, like it's too bugged or corrupt to function properly. But specifically for stories and not save files, I just play a few across the save. So if I get bored of dealing with my main family, I can work on their neighbor's storyline. Sometimes, a break is a good way to get your muse back for the old story, or to give you inspiration on how to end it in a satisfying way.
Like once, my sim was dating an man and they lived in an apartment, and she snuck out to Mt. Komorebi to earn money a snowboarder. I didn't vibe with the story, so I played with my sim's sister a bit. I liked the sister a lot more and focused on her family, and the first couple found a solid spot in the story as supporting characters.
So yeah. I think a good idea is to put a story to the side and try something else. Either absence makes the heart grow fonder, or you find something better.
