I'm Going to Become a Military Lady, Since It Seems I'll Be Abandoned After My Engagement Is Broken off

Celestia, the daughter of a prestigious family, is confronted by her self-proclaimed “heroine” stepsister with a prophecy: "You, my sister, will have your engagement broken, be abandoned by your father, and end up in a convent."
Determined to avoid this disastrous future, Celestia apprentices under her grandfather, a formidable general on the frontier, and resolves to live a strong life as a soldier.
…But somehow, she ends up catching the attention of the visiting crown prince?!