Which fictional "happily ever after" couple definitely breaks up, and why?

Rachel and Ross. Ross is a neurotic who doesn't actually no what he wants and is whiny. Rachel is a self centered brat who expects that good things should just happen to her, and again is whiny. Two horrible people who will never work out.

The original beauty and the beast always gave off some bad vibes for me if you think about it. Their relationship starting off as a non consensual kidnapping situation turned Stockholm syndrome romance is fucked up. Not even making the anti-protag Gustav a complete mysoginistic chud completely saintifies beast by comparison.
By the end even after saving bell the beast is still kind of an asshole. Not irredeemable he does good and tries to be a better version of himself, but his core personality flaws havent suddenly disappeared. The guy has some violent lashing tantrum tendencies, will probably gain back some narcissism as a restored prince, and maybe developed other mental emotional complexes from being a socially outcast man-bear-pig for several years. He needs midevil fantasy disney world therapy and they should probably have a few relationship counseling sessions if that mess is going to work.
- Gustav
  Gaston?
  Excellent analysis, otherwise. Also important to note that Belle falls in love with The Beast, which causes him to transform into... not-The-Beast. Like, is she going to be cool with that going forward?

A bit cheaty, but almost certainly every single fairy tale couple. They're all hinging the rest of their lives (and almost all of them are kids/teens) on the dual-threat of a rigid, idealistic vision of their future, and the very superficial understanding of this person they just met. Never a recipe for success.

Peter Parker and Mary Jane Parker. Because they had to fix some kind of time paradox I guess, a fix that had MJ marry an artist and have two more kids with him than she had with Peter.