sad dramas

These would be the pretty classic tragedies:
1 Litre of Tears
Sad Love Story
Sorry I Love You
Stairway to Heaven

A bit lighter non-tragedy ones:
49 Days
Secret
Scent of a Woman - romcom revolving around terminal disease. More of an optimistic sad, if you ever feel up for that.

I'm Sorry I Love You 
Autumn Tale  

( those two are the most TEAR JERKERS  Kdrama)

also :

A Love To Kill

Secret (though it has a good ending )