You can be enthusiastic for the viva and also feel nervous about it too. They’re not mutually exclusive states. Rather than think of ways to combat nervousness, could you think of ways to boost your enthusiasm?
To my mind, there’s lots of reasons to be enthusiastic about the viva.
It’s the final test! You’ve written a thesis! You are talented to be there!
Of course, I have a different perspective on the viva. The reasons I can think of to be enthusiastic might or might not help you.
So what would?