O don't think he was ever the same. Obviously Kay never had anything to do with him again, Connie may have tried to bring him back to NY for a time, but my guess is his diabetes got worse, and he basically turned it over to Vincent, returned to Sicily and lived out his days in misery. He died alone, his corpse being snifffed by a dog. Even Vito had a better death --- running around his garden playing with his grandson (although there's aother trauma for Anthony, who watched his grandfatherdie, his sisterdie, and more or less bore witness to the death of his uncle Fredo.