Doubters are more diplomatic than the naysayers, but underneath, both categories’ function is the same. Naysayers reject your ideas and visions outright without sugarcoating words. Doubters will initially support and cheer you on, but the moment you muddle somewhere, they do not miss nitpicking your shortcomings and claiming how they had foreseen your failure.
Perhaps it is the awareness of their inadequacies that makes them secretly wish for your failure too. You can certainly do without such friends in life.