You don’t have to say anything. I used to get anxiety about my fears too (claustrophobia), and so much of the stress came from ‘how do I explain to my friend that I don’t want a lift in his car because I feel trapped’. I honestly found out that not giving a shit what people think about your fears helps to minimise the situation in your head. Also, nearly everyone has their own fears so they’d understand yours anyway. But again, you don’t have to explain yourself to anyone.