I'm Just a Background Tutor, but I'm Doted on by My Favorite, the Third Prince

Mireina Emonsky, a duke’s daughter, realized at the age of ten that this world exists inside a story. From that moment on, she devoted herself to watching over the happiness of her favorite character—Cedric, the Third Prince—serving as both his tutor and childhood friend. Eight years later, at a grand dance party, Cedric is destined to meet Sherry, the story’s true heroine, and fall in love with her. Believing her role in the story is over, Mireina decides it’s time to find a partner of her own and tells Cedric she plans to focus on marriage prospects. But instead of supporting her, Cedric’s expression darkens—and to her shock, he proposes to her!? Based on the original novel: “It Seems I Raised My Favorite the Wrong Way — Why Is the Third Prince Doting on a Mob Noble Lady!?”

https://www.cmoa.jp/title/349175/