Fear Of The Unknown
There are two steps to removing fear or worry about unknown aspects of the viva.
The first step is to try to accept that your viva will be an unknown in some ways until you experience it. You won’t know how long it is, you won’t know what questions will be asked and you don’t know how you’ll feel until you’re there.
The second step is to build up a reasonable sense of the general expectations before you get to the viva. Read your university’s regulations, learn about other viva experiences and ask your colleagues about their recent experiences. These different aspects can fill in many of the blanks you might have.
You can have a good sense of what happens at vivas. This is a massive help both practically and emotionally, even if you can’t know the exact details of your viva until it’s your time.