Before your viva, work to feel more confident rather than less nervous.
The importance of your viva doesn’t change: you feel nervous because it matters.
Your confidence levels can increase though. You can reflect on your experiences. You can remind yourself of the successes you have created throughout your PhD. You can be sure of the improvement in your ability and knowledge. You can remind yourself that you did the work.
Work to feel more confident and you can feel less of the impact of nervousness.