Your passage is compelling and sets up an emotional and suspenseful story. Here are some comments and suggestions for improvement:
Strengths:

Strong Emotional Hook – Nana’s grief and struggle are well conveyed, making her a sympathetic character.
Intriguing Conflict – The tension between Nana and her new stepsister, as well as the resurfacing mystery, adds depth.
Smooth Flow – The narrative transitions well from her grief to the new family dynamic and the mystery.

Suggested Improvements:


Wordiness & Repetition:

"She can barely remember who she was before that fateful morning when she woke up and realized that something had gone horribly wrong."

This could be more concise: "She barely remembers who she was before the morning she woke to find everything horribly wrong."


"Stuck in the memories of her old relationship, she can't move on."

"Can't move on" is already implied by "stuck in the memories." You could tweak it for impact: "Trapped in memories of her lost love, she sees no way forward."





Clarifying Details:

"Too bad the only daughter of the Watanabe family isn't interested in being nice to her."

This feels a bit informal. Consider: "Unfortunately, the Watanabe family’s only daughter has no interest in welcoming her."





Tension & Stakes:

"When new details about her ex's disappearance are uncovered…"

Who uncovers them? Does Nana actively investigate, or do they fall into her lap? Adding a hint of agency would strengthen her character.





Revised Version (for clarity and impact):
**"Nana Yamashita has been a wreck ever since her girlfriend vanished nearly a year ago. She barely remembers who she was before waking up that morning to find something horribly wrong. Trapped in memories of her lost love, she sees no reason to break free from the monotonous loop of her tepid life in California—until her mother’s sudden marriage to a Tokyo businessman forces her to start over.
Thrown into the eccentric world of the Watanabe family, Nana must navigate the hostility of her new stepsister—a girl who, to Nana’s dismay, bears an uncanny resemblance to her missing girlfriend. And when new details about her ex’s disappearance surface, Nana faces an impossible choice: stay trapped in the past, or chase an uncertain future that may finally bring her closure."**
This keeps the emotional depth while sharpening the pacing and impact. Let me know if you’d like any other tweaks!