Sad songs are always good, no matter the mood (for me anyway). Here's a looong playlist (115 songs) of heart-breakers hand-picked by me (and a few I threw in that aren't necessarily my favorites, but give the playlist some depth and variety). Enjoy (or weep into your beer)!
And here's an even looooonger playlist (308 songs) of some of my favorite mainstream country songs from the 90s - the last decade in which anybody seemed to really give a damn about making commercial music mean something. You'll want to hit shuffle on this one, they're grouped by artist.