I don't want to downplay whatever trauma or emotional instability she has but I'm not buying all of it as a valid reason for her to be as starved for attention as she seems to be.
Her brother grew distant from her... as teen boys tend to do.
Her father passed away while overseas... tragic, but she seemed to have a stable upbringing when he was around and her parents doted on her by her own admission.
Everyone thinks she's upbeat and cheerful and she feels obligated to keep up that front... this could actually be a good angle for a character flaw as that kind of toxic positivity can play into societal obligations and expectations in Japan. But I don't think it works as an excuse to be clingy.
Oshiko was a lot more believable and sympathetic when she was apologizing to Miharu so I really hope author doesn't drop the ball here. Not exactly progress if we've simply gone from Nozaki angsting to Oshiko angsting for a far less convincing reason.