The Bookstore of Memories. What would you change if you could return to the past?

Jiwon suffers from insomnia and depression, she has lost the will to live, and the psychiatrist does not help her. One day, during a walk, she stumbles upon a strange bookstore. As she examines the books on the endless shelves, Jiwon realizes: her own memories are written in them. The shop owner offers the girl a deal: the opportunity to go back inside three moments of her past in exchange for time in the future. But when is the best time to go back? And can the past be changed?