Beyond Selidor

You must hate the majority of the instances of this trope in fiction because I feel like most of them take one of those two routes!

Unfortunately, you're right. I'll read pretty much anything if it looks like there's going to be a mortal/immortal relationship involved, which means I have read a lot of dreck over the years. I keep a sort of mental list of the stories that don't end in the way I dislike. Offhand, the ones that don't take either of the two routes include the Winterheart books, Spinning Silver by Naomi Novik, Sarah Rees Brennan's Demon's Lexicon series, most of Holly Black's books (she and I seem to basically share a brain when it comes to this trope), and Samantha Shannon's Bone Season series (although this is incomplete and I'm a little dubious about where the relationship is heading, given some things Shannon has hinted at on social media). There's also an Australian fantasy series called The Chaos Crystal about a suicidal immortal which almost ends the way I want, but takes a turn into the bizarre and absurd at the last minute in a really frustrating way.

Weirdly, although none of the relationships in Buffy are particularly happy or healthy, it handles this trope pretty well in regard to Buffy's relationships with vampires (although of course because the relationships are also being used as metaphors for other things, there is tension related to stuff other than the fact that Buffy is human and her boyfriends are immortal). I also feel that if Terminator: The Sarah Connor Chronicles hadn't been cancelled, it would have done interesting things with the trope as well.

What I really want to see (but for which I can't think of any examples off the top of my head) are stories about human beings falling in love with the deities that they worship, and the weirdness that would ensue, particularly regarding their own attitudes about religion. (I mean, you sort of get one half of this with Greek mythology, but it's not quite what I want.)

In any case, I keep seeking out these kinds of relationships in fiction, and very occasionally, I get what I want. I've also been fortunate enough to have a lot of people write fic for me about many of these pairings over the years!