These are the songs that put me a the 'sad' mood when i type:

ZIA - The way I am
http://www.youtube.com/watch?v=c7JDh9SKAY0

Beast - I'm Sorry
http://www.youtube.com/watch?v=uGQCdwnuQjs

Jiyeon - Rolling
http://www.youtube.com/watch?v=_sI9x_X4zqk

2AM - I did Wrong
http://www.youtube.com/watch?v=Im0hZGy37CY

T-ara - Good Person
http://www.youtube.com/watch?v=B1IEj_pO44g

IU - The Story only I didn't know
http://www.youtube.com/watch?v=ckHox5dFFwY

Sunny Hill - Pray
http://www.youtube.com/watch?v=Uive4DMqAdc

MBLAQ - I shouldn't Speak
http://www.youtube.com/watch?v=33ezPvdK2t0

If you are ever writing a part where there is a sad part, make sure to listen to these!