I'm Fat, and Everyone Has Mocked Me Since Childhood. Then One Day, I Found an App

At school, the heroine is bullied by a mean girl because of her figure. She unexpectedly discovers that her boyfriend is seeing the girl, and, spurred on by the cheating jerk and the mean girl, she comes across an app that guides her in seeking revenge. Along the way, she meets the hero. The app was designed by him, and with his help, she turns back before going too far and losing herself, successfully getting revenge on the cheating jerk and the mean girl.