she either doesn't/can't recognise the nature of her own feelings, or she's purely motivated by desperation and self-interest. I reckon if Shinoa intends to have a bittersweet ending, it will be the latter. But if not....
The reason I lean towards the former is because of the lengths the story goes to emphasise how isolated Eri feels from the experiences of her peers. Fuyu's assessment of her as a child is straight up that Eri was abrasive because she lacked any awareness of unspoken social rules (especially as a girl), and thus pissed everyone off constantly despite not actually doing anything. Cue being bullied viciously because of it. Eri herself frequently expresses confusion over how or why people act the way they do, and what's expected of her. She seems to have kind of gotten a grasp of how to behave generally (...by doing a personality 180), but clearly she's woefully underequipped to handle intimacy, and in no small part because she can't relinquish her death grip on Fuyu.
Not to armchair, but it honestly feels like she's written as ND, intentionally or not. She only seems to get on by following a bunch of "scripts" and/or doing what people tell her she should. Her freakout in c. 17 comes across as primarily driven by that feeling of social error: she's "wrong" to favour her best friend, but when she gets pushed around, betrayed, and cheated on by her other friend, she's "wrong" for how she reacts to that too. Everyone close to her feels like they're emotionally, developmentally leaving her behind because they're able to flip that switch and make romantic bonds their intimate focus as is expected of them at their age. Meanwhile, the only thing/person Eri chooses for herself and is adamant about is Fuyu and their friendship...which, again, everyone is telling her is wrong, childish.
It's very possible Eri clings so desperately just because Fuyu is the only person who doesn't place any expectations on her she can't live up to. And I guess a third (2.5th?) option is that she's ace or something. But the combination of her response to physical contact, how distressed she was by seeing Fuyu kiss Kisaki (plus how hard she rationalised it), and that narration from future Eri about how "she didn't know herself then" makes me think she just has a really strong mental block preventing her from properly identifying her feelings. I'm not convinced we're so close to the end, so I guess the drama's gonna proceed from that emotional immaturity and the chaos it's about to cause.
She's going out messy either way, like you can't just be doing that shit, girl.