Koushaku Reijou Lydia no Utsukushiki Ken: Uragirareta node Kochira kara Konyaku Haki Sasete Itadakimasu

Since she was just a child, Lydia was determined as the daughter of a Marquis to do everything in her power to support the royal family. However, everything seems to be changing before her very eyes as her fiance Rogerio begins showing romantic interest in a lowly noble aide to the princess. While ignoring their duties, the two seem to be getting awfully familiar... and yet Lydia seems to only be treated as an antagonist when confronting the situation. Feeling betrayed, her manners and patience have reached their limit...! (Source: MangaPlaza)