Turner told Fortune Magazine he is concerned that his huge losses will mean that the philanthropic causes he supports will suffer.
Fortune, which is part of the AOL Time Warner conglomerate, noted that Turner will step down as vice chairperson of the corporation at Friday's annual shareholder meeting. He was quoted as saying the move comes "in disgust," but without elaboration.
But when Turner was not consulted about a change in top management at CNN, "it was the final straw," he said.
Referring to former AOL Time Warner chief executive Gerald Levin, Turner said, "The biggest mistake I made was trusting Jerry too much."
Turner initially sold his TBS broadcast empire - CNN, TNT, the TBS Superstation, Turner Classic Movies, the Cartoon Network, New Line Cinema, and several sports teams, including the Atlanta Braves - in 1996 to Time Warner.
That became absorbed into AOL Time Warner when America Online used its high stock value to acquire the media giant in 2000.
The magazine estimated that Turner's stock holdings declined from $10.7bn in early 2000 to $1.4bn as a result of the 81% slide in the value of AOL Time Warner.
Because he sold or gave away some stock during that period in the interim, his actual losses down were pegged at $8.5bn.
"I'm the stupidest person in the world not to have sold earlier," Turner is quoted as saying.
"Over 90% of my liquid assets were in AOL Time Warner. No one with substantial wealth should ever concentrate their wealth like that, particularly in a company where they don't have control."
Turner's net worth, which includes 768 000 hectares of land and sizeable cash holdings, was estimated at $2bn.
But because he still owes more than $600m to his UN Foundation, he is down to his last billion, Fortune said.
"All I was doing with my multiple billions was giving them away," he moans. He has scaled back his philanthropy, which, he says, "gives me a great deal of pain because the needs are greater than ever."
Asked about his future plans, Turner alluded to the final scene in "Gone with the Wind, saying, "Right now I'm kind of like Rhett Butler walking out the door on his way to Charleston to look for a more noble life."
Turner cashed in $790m from selling AOL Time Warner shares, and said he would invest it "prudently and conservatively."
According to Fortune, he is considering Treasury bonds, stocks, and more land. He is committed to putting at least $40m into his Ted's Montana Grill restaurant group, which pumps money into bison ranches.
Turner, who will be 65 later this year, recently separated from his strong-willed, 53-year-old French girlfriend, Frederique Darragon, according to Fortune.
"I'm like a little baby left at a doorstep, needing a woman to take care of me," he says.
But Turner is not amused by his onetime media rival Rupert Murdoch, who now appears to have a massive empire in place with Fox broadcasting and film and a host of other media firms worldwide.
"Rupert Murdoch is the most dangerous man in the world," Turner was quoted as saying.