In My Past Life I was the Cold-Hearted Emperor, in This Life I'm a Young Girl summary:

"Juliana, your fiancé has been decided.” At that moment, the frail daughter of an earl awakened to a shocking truth—within her resided the soul of the infamous "Cold-Hearted Emperor" Julius. A man whose ruthless conquests left behind mountains of corpses and an empire under his iron rule. Now, reborn as a delicate young noble girl, Julius finds himself in a seemingly peaceful era. But beneath the surface lurk corruption, poverty, and disease, all exploited by scheming villains. With the wisdom and might of a former emperor, Juliana will not stand idly by. The young lady who once ruled as a tyrant is ready to cleanse the empire once more—this time, with her tiny hands. A dark yet thrilling tale of power, justice, and a battle against the hidden evils of the empire begins!