KEN TO TIARA TO HIGH HEELS: KOUSHAKU REIJOU NI WA EIYUU NO TAMASHII GA YADORU @COMIC

The kingdom's hero, Ortus (♂), dies in battle after failing to fulfill the promise he made to protect the kingdom for the rest of his life. When he wakes up again, he finds himself reincarnated as Seretina, the duke's daughter! Even though she's a beautiful girl on the outside, she still has the same determination to become a knight to protect her country in this life. However, besides being sickly and fragile, she is also faced with the harsh duties of a lady. She has to learn manners from her horrible mother, marry a man she doesn't want to marry, and many other challenges that will keep her from becoming a knight. On the way to the royal castle, a demon attack reveals her true power! A new heroine is born, with the strength to slay demons with a single blow, and the peerless beauty to captivate high society! She’s pretty on the outside, but fearless on the inside! The chivalrous heroic fantasy of a young woman begins!

(Source: TO Books, translated)