What if your examiners ask you a question and you go blank?
What if you forget something?
What if your examiners don’t agree on the outcome of the viva?
What if you arrive late?
What if you don’t know the answer?
And so on. The consequences for some of these situations seem bad. Some of them are easily coped with in the moment (if you go blank, think more; if you don’t know something, discuss it with your examiners), and some aren’t. But none of these will necessarily happen in the viva. Some of them are not likely at all.
You can’t always control how you feel, but rather than obsess over what-if scenarios, try to give your attention to preparation that helps.
What if you invested your time and energy in being prepared for the viva? What would happen then?