I really loved this K-drama and generally the way Korean movies picture first love stories. Such K-dramas bring back my own memories.
What most of the people disappointed with the ending ignore is the passing comment made by Hee-Do's mom to the adult Hee-Do about having recently met Yi-Jin. I do not recall what episode was that in.
I prefer to think that the ending is actually open and they manage to get back together eventually.