Signal AU idea... just gonna leave this here cause I barely have time to write but I love signal so maybe I'll come back to it one day (unless someone gets to it first!)

Mina feels guilty every time, and so she keeps making Momo forget. She always tells herself she'll never do it again, but somehow she always finds herself in the same situation--sometimes she initiates, sometimes it's Momo, sometimes it's an accident. The ending is always the same, leaving Mina a sniffling mess for her roommate Chaeyoung to clean up. 

One time, Momo doesn't forget. (or does she? Did Sana somehow tip her off?)