One possible theme could be self - reflection. Sakura realizing she's a 'bitch' means she's looking at her own actions and behavior critically. Another theme might be redemption. If she realizes she's been wrong, she may try to make things right.
The fanfiction might explore the theme of character growth. Sakura's realization is the first step towards changing and becoming a better person. Also, the theme of relationships could be there. Her 'bitchy' behavior might have damaged some relationships, and now she has to face the consequences and try to repair them. It could also touch on the theme of self - acceptance, as she has to accept this negative side of herself before she can move forward.