Angnyeo 18-se Gongnyakgi

Commander Liza Mary of the Imperial Knights returns victorious to the Empire of Phaeton after a long war to subjugate an enemy kingdom. But little did she know that her beloved fiancé Illian would leave her for Flora, the princess of the fallen enemy kingdom… After drinking herself into a heartbroken stupor that night, Liza awakens to the modern world in the body of timid high-schooler, Do-Eun. Puzzled and confused, Liza seeks to find out what happened and return to her own world, aided by Do-Eun’s friends Ian and Lora, who look suspiciously like Illian and Flora!

(Source: WEBTOONS, edited)