Before You Discard Me, I Shall Have My Way With You (Manga) Vol. 1

Agns, the daughter of a duke, is in love with crown prince Lucilleur, the man she's been betrothed to since they were both children. But before they can wed, Agns is hit with a bombshell: Lucilleur doesn't return her feelings and wants to break their engagement so he can marry his true love, a different noblewoman with a sweet and innocent demeanor. Agns is more than just heartbroken - she's angry. Her 'sweet' rival might not be sweet at all, and her beloved prince has wrecked her future. This means war... both in politics and in bed.