Her being 'scared and emotionally closed off' doesn't seem to be anything other than an excuse at this point.
They are 'dating' for almost half of year even in the start of the story, yet she doesn't give her attention. She always chooses club over her, can't even hold hands or call her by name while being very close with other girls, especially Yuki chick. She ended up bringing her whole damn club to their supposed festival date and just went to spend some time with Yuki in the middle of it. And she started to try to act like a proper gf only after she forgot their half-year anniversary (the reason was the club yet again, btw).
Even in previous chapter she just let herself being pulled off by Yuki and just sat there, looking how she eats. And, like, at this point, from Nanase's perspective, Yuki knows, that they are dating. She could just refuse, yet she didn't.
I'm not trying to say, that she's the only one at fault here ofc, Yuni is pretty fucked up, but Nanase is by far not an innocent in this situation. Sometimes it feels like Yuki is as important to her as Yuni.