I never been nervous of flying. But soon after my second child was born, I had two very near mid air collisions in the space of 3 days which made me question what I was doing flying with a young family to support. And after a lightning strike which gave me a big electric shock, I have since always been nervous when flying near cumulo-nimbus clouds.
I once had a student, who was reluctant to handle the aircraft other than very gently. He had stated that his aim was to go onto heavies and not fast jets. But one day, I noticed his hands were shaking as we were walking for the first spinning sortie and I realised he was very nervous. And one day when I was DI, I watched one of his solos on the radar. He just flew around in gentle level circles until it was time to come back to do some circuits. One day, I mentioned to him that if he didn't think it was for him then there was no shame in withdrawing. He seemed relieved that I has realised his problem, but he said he wanted to carry on. In the end, he failed the end of course check and decided that he didn't want to do a re-take. I saw him a few years later where he was very happy as a supply officer.
One of my colleagues in my current airline had previously voluntarily withdrawn himself from TWU through nerves, leaving the RAF. He was a nervous airline pilot too. Mind you, he wasn't the luckiest guy. If anyone was going to have an engine failure at V1, it would be him. He quit flying and I last heard he was a maths teacher in a secondary school in Glasgow - which doesn't strike me as being very safe!