“I have nothing left to live for!” With those heartbreaking words, ROBIN WILLIAMS, who lived to make others laugh, confided his deep personal despair to a close friend.

The National ENQUIRER has learned exclusively that the beloved funnyman, in addition to battling depression, had been battered by a painful string of health issues – including a secret nervous breakdown and dramatic weight loss.

And while his wife disclosed he was fighting early-stage Parkinson’s disease, The ENQUIRER discovered that some friends believe he was also fighting cancer.

His personal hardships finally overwhelmed him, multiple sources told us, and he took desperate measures to escape the pain.

In the last photo of Robin – obtained by The ENQUIRER – the troubled star appears frail and gaunt… a grim-faced man alone in a crowd.

It was snapped at a local art gallery where he and third wife Susan were surrounded by patrons. “But Robin appeared frightfully alone,” said a source.

“No one knew – not even his wife – that he was going to kill himself just one night later.”

On Aug. 11, he would be found dead at 63 – after hanging himself with a leather belt in a bedroom at his Tiburon, Calif., home.

In the days prior to Robin’s untimely death, confided a close source, “There were whispers he had cancer. He had lost about 20 pounds in recent weeks and in addition to the Parkinson’s, he must have felt like a death sentence had been placed upon him.”

FOR the full heart-wrenching story – it’s only in the new ENQUIRER – on sale now!