Ex-Wife Stop (divorced and then they meet again)
Together with Me (Main leads are married but something happens and the ML disappears)
Alchemy of Souls - maybe even the title is a spoiler.
My Bossy Wife , My Bossy Wife Season 2 - (separated for only a few months though)
The Glory (they meet he is instantly attracted to her, they are not together as a couple but he helps her and then she disappears. Romance is not the main genre though.)
Till the End of the Moon (it's an enemies to lovers plot. Even though both fall for each other, they separate at one point for years.)
Fireworks of My Heart - if you don't think late teens is childhood then this might match up. FL and ML were classmates in high school (final year I believe) and get separated when FL's mother doesn't approve of them being together and forces her to attend university abroad.