you should write a manga you got good thoughts in that noggin. (๑•ㅂ•)و✧
the manga that came to mind was https://www.mangago.zone/read-manga/shounen_no_kyoukai/
it’s not exactly the same, but yeah the characters don’t end up with their fated mates
https://www.mangago.zone/read-manga/shiawase_ni_michita_yoru_no_niwa/
I think this one meets your criteria. They weren’t in a relationship in the beginning, but he did meet his fated pair and was not allowed to be with them, then he met the ML and fell for him instead.
takatora kun to omegatachi...not exactly that but its a more nuanced omegaverse with commenntary on sociopolitical aspect of gender.
https://www.mangago.zone/read-manga/romantic_lament/ >> (apparently twins share fated mates and choice is a main point here)
Funnily enough I am a writer but just basic level stuff, I draw a lot of inspiration from reading hehe hope to one day release something ٩(๑❛ᴗ❛๑)۶
Thank you for the recommendations everyone
ヾ(☆▽☆)
More specifically stories where they define the ABO lore like example they are in relationship and meet their “destined partner” but choose the one they really love and want to be with but the story still has that beautiful agony of “what ifs” yet ends happily for the main couple or even tragic ones lol idm