Childhood Friend Romances

In time with you - this is the perfect drama to watxh. Bolen chen totally stole my heart all over again in this drama

Reply 1997 - the girl did not officiall date anyone but she has a huge obsession with someone else and his older brother started courting her.

Reply 1988 - she was 'in love with being in love'

Proposal Daisakusen - has a bug fantasy element

My love eundong - she is married with someone else

Seventeen (webdrama) - there are 2 stories here, the main couple sadly did not end up together, the secondary couple had your plot

2 outs in the 9th inning

Because it is the first time - sadly this not end up with the guy getting the girl, but somehow the ending was painfully beautiful

Others: The time they were not in love (the messy korean version of in time with you) and One more happy ending

I Hear Your Voice

The best two for this trope (my absolute favourite) are, as Czakhareina listed, In Time With You & Reply 1997. If you prefer more mature characters living out completely realistic experiences with no sugar coating & lovable but sometimes messy leads, the former, with Ariel Lin & Chen Bolin should be your first watch. If you want a genuine coming of age story that is wisdom and truth mixed in with a beautifully bittersweet fairytale quality, the latter, R97, is a must.