Seiryaku Kekkon kara Nogareru tame ni Kakusaku Shiteitara Itsunomanika Akuyaku Reijou ni Natteita Ken

Nina, princess of the kingdom of Serene, is actually a former Japanese college student in her past life. She has reincarnated as the villainess of an otome game, with nothing but bad endings ahead of her. To change her fate, she disguises herself as a boy, rigorously trains in swordsmanship and magic, and sets her sights on a happily single life with zero marriage prospects.

https://www.cmoa.jp/title/356900/