I can really relate to the feeling that you feel like the person you used to be died. I feel the same. I too first became depressed (or was aware of it) when I was 14. I used to be so happy before this and couldn't understand how I had changed so much. The only logical solution to me was that the "old me" had died and left behind this new version that couldn't feel joy anymore. I know this isn't true. In a way, it used to be a comfort to think this. It meant there was a reason for me changing. Because I couldn't understand it otherwise.