Hmmmmm, I'm gonna have to think about this...right off the top of my head, I can think of a few songs that deal with the subject of addiction. I don't think addiction is a mental illness though..
This is too easy: https://www.youtube.com/watch?v=pycizbGj0Ik https://www.youtube.com/watch?v=V6Dfo4zDduI https://www.youtube.com/watch?v=QQBttKoetqo C/S, Rev J