Sorry to hear about your sadness

Rachel, I think what you're going through is a universal human emotion. I think whenever one recalls the past, those emotions may be relived. Music, photographs, journal entries...etc, have ways of triggering memories and emotions.
What might be helpful is to think of why that song made you feel depressed, confront it, accept it, then start to move on in your life. Or think of happier things. I love listening to KPOP!! If you're interested, listen to 2NE1's "Don't Stop the Music." I have no idea what they're saying, but it's totally my jam right now...! Totally puts me in a dancing mood, and any dancing mood is a good mood
Audree