This Is My Mansion. Could You Leave Now?

Marissa, a 17-year-old daughter of a marquis, plans revenge on her uncle's family, who have taken over her mansion since her parents died. On her 18th birthday, her title will be returned to her... Until that day, she has endured bullying at home and at school, determined to exact revenge on those who have oppressed her.

Then, her fiancé, Delior, unilaterally breaks off their engagement. She learns that Delior loves her uncle's daughter, her stepsister, Roxanne, and believes her to be his soulmate. Marissa, who cannot bring herself to like Delior at all, welcomes the break-up.

"I'll give him a proper revenge along with my uncle's family!" she vows to herself, but contrary to her strong will, her body has become emaciated from years of oppression. Unable to bear seeing her like this, Reid, a classmate from school and the country's third prince, approaches Marissa after the engagement is broken off...

(Source: Alphapolis, translated)