Any kdramas where the main character was a complete jerk then something happens that makes them try to be better
In most of these there isn't exactly a moment where "something happens that makes them try to be better" - it's a bit more gradual than that:
Kiss Me Love by Chance 2: A Chance to Love SOTUS Theory of Love U-Prince: The Handsome Cowboy Devil Beside You Easy Fortune Happy Life Fall in Love With Me It Started with a Kiss Marmalade Boy Moon River The Prince Who Turns into a Frog The Rose Why Why Love Successful Story of a Bright Girl The Greatest Love You're Beautiful Hana Yori Dango
