let us know where the volume breaks are, and we can fix it
Every nine chapters, at least for the first two volumes.
I dunno. Both Saeko and the ex-not-girlfriend look pretty mature and down to earth. "It doesn't work? Well, let's get over it and look into the future!"
There's nothing mature about bottling everything up and constantly lying; or about ghosting your friend for a year because you might have feelings for them, only to suddenly come back to make a pass at them.
But Miwa is constantly on the wishy-washy side, unable to take a stand. I understand she lacks self-confidence, and people like that are a thing, but I can't empathize with that kind of characters. She strikes me as lacking maturity.
And if everyone is "a bitch" to Miwa, it's precisely because of that: she doesn't have a presence, she's completely passive and doesn't communicate properly.
Miwa has plenty of room to mature, but she made strides during the music club trip, whereas Saeko has only gone further down the rabbit hole of denial.
As far as communication, Miwa has repeatedly asked Saeko about her past and what's going on, but Saeko has put on a brave face every single time. It's difficult for her to dig any deeper when she knows that she has annoyed Saeko by worrying to much about the little things before. Miwa also told Saeko that someone she had a big crush on (she clearly still harbors some feelings for her, but she was probably not completely aware of that herself) would be at the reunion.
She "got over" her old crush because she saw a pic of her with a guy, but now that she know it was just a fling, she's suddenly agitated.
To be fair, it would be hard enough nursing a crush on (1) another woman (2) who lives far away and (3) has been ghosting you. It's not surprising that it would take something small for her to 'give up' on Shiho after that.