Robin Williams -- Rehab Came 'Too Late' For Survival

Robin Williams was in such a deep depression when he went to rehab last month ... people around him felt it was simply "too late" for him to recover.

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."

We're told Robin had been receiving "spiritual treatment" ... lots of meditating.

Robin had been dealing with depression for most of his life.