Dramas where the FL is the ML's first love, but they don't get together until the ML gets a second chance years later (in some of these the FL does not date someone else in the meantime):
- I Need Romance 3
- Weightlifting Fairy Kim Bok Joo
- My Secret Romance
- The leads meet as adults, but the ML has been a playboy up until that point
- They have a one night stand (her first) and he falls for her / she's his first real love, but she disappears the next morning and they don't meet again for three years
- Doctors
- In the beginning the ML is the FL's high school teacher, so he doesn't pursue a relationship with her, but he gets a second chance when they become colleagues years later
- The King's Woman
- When I See You Again
The ML and FL have a childhood connection and based on your interpretation the FL might be considered the ML's childhood / first love:
- It's Okay, That's Love
- Just Between Lovers
- The King in Love
- General and I
- Strong Woman Do Bong Soon
- I Hear Your Voice
- the leads first meet when the ML is a kid, and the ML is still in high school during the drama
And I second In Time With You.