Watashi No Hatsukoi Wa Hazukashisugite Darenimo Ienai summary:

Chiaki is handsome, academically gifted, athletic, and born into a wealthy family with a beautiful sister. Despite his seemingly perfect life, he faces a persistent problem: he's never been lucky in love and struggles to capture the attention of girls. Determined to turn things around in high school, he wakes up one morning to a surprising transformation—he has turned into a girl!He is shocked to find out that he has lost the appearance he was so proud of, but when he looks in the mirror, he sees a very cute girl in the reflection. He immediately goes to show it off to his twin sister Kaede, who happens to be very popular with the girls, but there is also something very strange happening to her...(Source: Kadokawa, translated)