The viva’s not the first time you’ve had to respond to questions about your research.
The viva’s not the first time you’ve been asked about your contribution.
The viva’s not the first time you’ve really had to think hard about something.
And it’s unlikely that your viva is the second time you’ve had to do any of these, or the third, or the fourth.
Again and again throughout your PhD, in small and big ways, you grow, you learn and you become better than you were. Again and again you demonstrate that you are a talented, capable and knowledgeable researcher.
In the viva you have to do it again. One more time, but with all of that experience behind you.
You can do it. You’ve done it before, you can do it again.