Don Cornelius Was In Extreme Pain Before Death

According to the report, the "Soul Train" legend had been experiencing seizures as a result of an aneurysm he suffered 15 years ago.

But Don's health took a dramatic turn for the worse in the last 6 months of his life ... and he became "very depressed about his failing health."

Don called his son at 3:00 AM on February 1 ... and warned his son, "I don't know how long I can take this." Don's son told him he'd rush right over ... and Don said he'd leave the back door open.

When Don's son arrived to Cornelius' Los Angeles home, he detected an odor of smoke and saw Don seated in a chair with a pistol in his right hand.

Don's son called 911 ... and paramedics treated the 75-year-old for a bullet wound to his right temple.

He was transported to a nearby hospital ... where he was pronounced dead a short time later.