Here are several suggestions:

Toutes les bonnes choses ont une fin.

Il n’est si bon cheval qui ne devienne rosse.

Il n’est si belle rose qui ne devienne gratte-cul.

With your comment about
“[something] you might find yourself saying to yourself with a sigh of disappointment after having just caught your partner in an unattractive moment”,
in mind, …
… I can perhaps imagine myself muttering (granted, after not 100, but 38 years of marital bliss):

[God, she doesn’t seem to realize that] there are limits to
everything! …

… as my much better half drones on about this or that while I’m engrossed in one of my favorite Soap Operas.

Although perhaps not fixed expressions/idioms, literal French translations of the bolded part of the above muttering might include:

“Il y a des limites à tout”
or
“Tout a des/ses limites” …

… which, when muttered all alone, wouldn’t necessarily imply that whatever limits there are have been surpassed (or even, once surpassed, what the consequences might be), but in the right context, they would establish that even (especially?) old, enduring romances, or at least the partners therein, do have limits, especially to their patience.

Though not a phrase or a proverb, there is this common word: un tue-l’amour. It refers to a habit or a behaviour that will instantly put off your partner or your (potential) lover.

So you may have something similar to your idiom in those contexts:

Je déteste quand il met des chaussettes de tennis avec des sandales ouvertes. C’est tellement peu élégant que c’est un tue-l’amour.

Il ne se brosse jamais les dents. Cela devient un véritable tue-l’amour.