i want a protag who's already in a relationship (married preferably) but the husband/wife doesn't really care about the marriage, so the protag is tired of trying. YK like when the mcs try to make it work but are continuously disappointed bc of the other side. Yea, so like that but they don't end the relationship bc of something (idrc if they end the relationship too much like if the lover is trash leave, but if they can make it work then yea). So yea the protag is tired of trying, and the lover notices that the protag doesn't care anymore and goes through that whole phase of 'u never know what you had until it's gone'. edit: i thought abt it some more, can also be chasing/courting, no need for a relationship. just the trope of the 'u never know what you had until it's gone' Please and thank you
Ookami Shounen wa Kyou mo Uso wo Kasaneru A manga, but it fits very well. For the spoilers, it's one of my favorite. That ending I loved
https://www.novelupdates.com/series/yumemiru-danshi-wa-genjitsushugisha-ln/ It's not finished but it prob will eventually have a HE i guess. apart from that, it fits what you're looking for in regards to It's the whole basis of the plot.
The Tragedy of The Villainess Her fiancé spends more time with the saint and does everything to her. MC is disappointed until something strong happens and her fiancé wants her back but she doesn't want him anymore. Remarried Empress