Stress and nerves are not penalised in the viva. If you’re anxious or hesitant you don’t get extra questions or corrections. Your examiners will not think less of you if you’re worried on the day.
The real question is what are you going to do about your nervousness for your own sake? Do you need a couple of butterflies in your stomach to help you? Or would it be good to banish your anxieties entirely?
It’s not enough to say “I feel nervous” or “I feel worried” for the viva. There are two questions that follow when you feel something negative about your viva. First, why do you feel that way? Second, what are you going to do?