Sources who were around Robin at the time he decided he needed rehab tell TMZ ... he was in a deep, dark depression. The people we spoke with confirm what Robin's people told TMZ when we broke the story ... he had not fallen off the wagon, but was struggling to center himself.
One source says, "Just before he checked in [to rehab] it was obvious ... he had not gotten treatment for so long he was too far down the road."
Another source says he had been "internalizing all the pain of addiction and it was obvious to anyone who was around him."