Hmm, at first it sounded like this http://www.mangago.zone/read-manga/kanawanu_koi_no_musubikata/ from your description but now I think it's this one.... http://www.mangago.zone/read-manga/shounen_no_kyoukai/
If fated by red string i just remember this one lol
http://www.mangago.zone/read-manga/akai_ito_no_shikkou_yuuyo/
Maybe this one?
http://www.mangago.zone/read-manga/shounen_no_kyoukai/
Kashikomarimashita destiny, the side story is close I think
Im looking for this BL where the main characters met thier fated pair but they refused to be with them because they already chose and promised to love thier first love.
The other pair was heartbroken but they all eventually found happiness.
Do you know any manga that resembles this on?