HELP ME FIND THIS MANGA

OK, so basically it’s about this girl and her boyfriend, they both attend two different schools, one day as she overhears or sees that he has a girlfriend, And he’s basically fake dating the girl from his school so that his older brother who is a teacher can see her, if that makes sense, She ends up, just ghosting him and never speaking to him again, and then one day in the future they reunite, but she’s a completely different person. She’s more monotone and not like her cheerful self please help me I’ve been searching for the past few months and at this point, I don’t even know if it’s a real story or a dream that I had.