I just screwed up and posted to the original Saddest Song instead of letting it fade off into an unused thread . . So I will repeat myself here.
Don't know if it's the saddest song, but pretty darned close, "I Can't Make You Love Me" by Bonnie Raitt always gets me. The main line that says it all is:
"I can't make you love me if you don't, you can't make your heart feel what it won't"