Konyakusha ga Uwaki Aite to Kakeochi Shimashita. Ouji Denka ni Dekiai Sarete Shiawase na node, Imasara Modoritai to Iwaretemo Komarimasu.

Amelia is a studious young countess and water mage. Her fiancé, Reese, is the second son of a marquis and wields earth magic, a desirable skill for the agricultural domain that Amelia’s father rules. The couple grew up together and has always gotten along well, so their happy future together seems certain. Yet after Reese enters the Royal Academy of Magic, Amelia loses all contact with him. Although she assumes he’s just busy with his studies, when she enters the academy a year later herself, her classmates inexplicably ostracize her as Reese continues to avoid her. Throughout this confusing ordeal, the isolated and anxious Amelia finds comfort in the kindness of Sarge, the fourth prince, who seems to have taken an interest in her...
(Source: J-Novel Club)