The ending is disappointing, yes.
In volume 4, Aoi becomes impatient about not being able to spend time with Sahoko due to her other friends, so she confesses to Sahoko with a kiss. Sahoko is overjoyed, believing they've become a couple.
When Sahoko goes to Anna's house alone to ask Anna about dating advice (not knowing about Anna's feelings for her), Anna tries to forcibly kiss Sahoko with the excuse that such physical contact is what "romance between girls really means". Sahoko is repulsed by Anna's touch and pushes her away -- Anna plays off her actions as a joke, but Sahoko is shaken, questioning if her feelings for Aoi are really physical/romantic love
Aoi continues to try to escalate her physical connection with Sahoko out of jealousy towards her other friends. Sahoko initially tries to suppress her discomfort, but has an outburst and finally refuses Aoi as well. Sahoko concludes that her feelings towards Aoi are not romantic, which then leads to Aoi breaking off their relationship, because Aoi can't bear just being friends. This sets up the lack of contact between them for the rest of/after highschool, leading to the class reunion scene which gradually progressed throughout the story.
In the "present" time years in the future, during the class reunion, Sahoko is trying to get ahold of Aoi because she wants to "confirm her feelings". Despite their separation, Sahoko has been thinking of Aoi this whole time.
Aoi initially refuses to contact Sahoko until she hears that Sahoko will soon be leaving on a student exchange program to America. She has also been thinking about Sahoko all this time, so she rushes to meet Sahoko.
Sahoko reveals that her exchange program is only for a month, so it's not a permanent separation. However, when Aoi then tries to take off, Sahoko invites Aoi to a love hotel.
Sahoko and Aoi have sex in order to "confirm their feelings". Sahoko is depicted as enjoying it, finding it deeply emotionally fulfilling.
However, in the morning, Sahoko wakes up to find Aoi gone, with just a note left behind. In the note, Aoi admits that she was mistaken in forcing Sahoko to choose between love and friendship/admiration. After so many years had passed, she could no longer find meaning in waiting to hear Sahoko's decision on her feelings. Rather, she was simply glad for having met her and grateful that Sahoko tried seriously facing Aoi's feelings.
The note concludes with "Be well; thank you for everything" so Sahoko runs out of the hotel and yells the same words into the sky as well.
Afterwards, the story fastforwards to Sahoko's last year of university where she's shown voicecalling with Anna. Sahoko and Aoi are implied to still be always thinking of each other but never to actually meet. In particular, back during the class reunion Sahoko had already been shown as having cut her hair short like Aoi's in highschool, while Aoi had grown hers out like Sahoko's.
The "message" is that, although the emotion Aoi and Sahoko felt for each other was both admiration and love, in the end admiration won out, so that -- with Aoi and Sahoko having become different people during their separation -- their distance is "necessary" for them to continue holding on to the connection they had in highschool.
