Thankfully I've been fortunate enough never to have been through what some of you seem to have suffered. I went through a bad period in my life, years ago now, where I just felt completely empty but it was very much reacting to circumstances and not what I'd call a mental illness. Thankfully it passed after a month or two. My way of dealing with it was finding a new job in a new city, I just needed the idea that I was going to work with new people in new places and that unpredictability and a fresh start kicked me back into life again.
I don't really know what to say to help anyone who is suffering. All I know is when people told me to 'stay positive' I wanted to tell them to f**k off, such was their lack of understanding of the situation. Good luck I guess!