Negative thinking. Happened in the past?

My father has a sinusitis problem which was diagnosed in 1992.During that time he went through all kinds of medication and went to several doctors who give him different medicines before the problem was diagnosed. One of the doctors to whom he went( he was a close family doctor but of very short temper) gave him 15 days medicine but it did not work and so when he went to him again he started yelling at my father and said that he will be given shock treatments as he was very very ill tempered. He said it very rudely and since then whenever my father gets any head pain due to sinusitis he gets very depressed and thinks of that same thing that doctor said which was absurd which he knows well but it makes him depressed every time and he gets really worried about that shock thing. He also went to a psychiatrist for this depression treatment but he is not able to overcome it. What should be done? How can he stop thinking about what happened in the past? Please I need help!