Scott said it himself years ago in that Esquire interview. Heroin led him to feel like he was holding onto his former self dangling over the edge of a cliff, and he could feel his former self slip away. Over the years, with the relapses, and the drinking and abuse of meds, constantly trying to numb himself rather than deal with the mess his life had become, I think his former self finally lost grip and fell off that cliff years ago...
Not to mention, while he paraded around with the Wildabouts, faking his big excitement about it being his first project since Core he was so excited for, while singing and acting indifferently on stage, like he couldn't be bothered. He was actually talking to Chris Kushner last month and really wanted VR to get back together (I guess it signified a period of stability in his life, almost like a cry for help). That and what Mary said in this letter about Scott calling her up over the last few years, at night upset he couldn't get out of his bad situation surrounded with bad people and choices. It seems to me like any glimmer of the real Scott, like Mary said, just gave up somewhere along the way. Perhaps there was but a glimmer left, but now it's gone forever, and that is the biggest tragedy.