Any rom drama where the SML or SFL misidentifies the ML or FL as their childhood sweetheart and then wastes a ton of time and feelings trying to hook up only to realize he/she was all wrong and ends up in a dilema. However, the lead shouldn't be confused about the identity of the other person, so basically I'm guessing the SL keeps it a secret he/she thinks they know them. 

I Have A Lover. The SML thought the FL was his childhood sweetheart.

Maybe Renascence but she wasnt SFL

Spring Waltz.
But it's oldschool drama, so there is some dragging and cliché scenes sometimes.