Heptagon

Overview

Nana, an apathetic 24 year-old with nothing much in her life, suffers a car accident and wakes up in her 14 year-old junior high body. As she lives through each day, she starts to remember things she buried deep in her memory and she can now not remember being involved with her older sister, whom she deeply admires, and her sister's first love, a senior at her school.

Source: AniList