I know what makes me feel uncomfortable. Sometimes it’s possible to avoid things outside of my comfort zone; I’m self-employed so I have a certain degree of control about the kind of work I do, or the conditions I work in. Sometimes stepping out of my comfort zone is necessary though. For those situations I’ve had to figure out how I can best proceed; I’ve figured out how I can make the most of those situations even thought they’re not comfortable.
In some cases, like public speaking, I’ve even come to like something that was previously way out of my comfort zone!
It’s useful to figure out your comfort zones so you can work well, and especially useful for PGRs nearing the end of the PhD process. If the thought of the viva makes you feel uncomfortable then I think the best thing you can do is stretch yourself in advance. Stretch by presenting, by discussing, by working to build your confidence. Find more ways to practise, even small ways to get more experience and learn what you can do to make the situation better, more comfortable. Like me, you might even find a way to make the process more enjoyable for you.
Perhaps your viva will be closer to your comfort zone than you expect.