Read The Young Lord of House Finlanz, Finrantsuka no Osanai Ryoushu, フィンランツ家の幼い領主 Manga
My name is Lizbeth, and I am a 7-year-old who will use the knowledge from my previous life to develop my territory! Lizbeth was born as the daughter of Margrave Finlanz. She was married off in a political marriage to Crown Prince Reinhardt as his 10th concubine. To distract herself from her loneliness, she exchanged letters with friends from other countries. However, she was falsely accused of a crime and imprisoned in an underground prison. With no one listening to her pleas, she drew her last breath alone, but… “I’ve returned to the past—?” When she came to, she found herself in her warm bed, back to being her happy 7-year-old self! The remote territory lady’s coming-of-age fantasy starts with a loop! Hip Hip Hooray~!