The voice saying “you can’t” usually sounds certain. Certainty isn’t proof.

Fear speaks in a very assured tone, which is the whole trick. It does not say this might be difficult, it says you will fail, and it says it with no evidence and no hedging. Confidence of delivery gets mistaken for accuracy, especially when the voice is inside your own head and sounds like your own judgement.

When the flat verdict arrives, ask it for evidence as you would ask anybody else. What exactly is the prediction based on? Usually it is one old memory. Then do a small version of the thing and see which of you was right.