When have you been stretched during your PhD? What projects or work have you done that was a challenge to your skills and knowledge?
I think it’s unhelpful to view the PhD (and the viva) as the most awesome and incredible challenge in the world ever™ – but it’s also unhelpful to not recognise the growth that happens through the challenges someone faces along the way.
Reflect on the challenges. Reflect on the stretches – when they happened, why they happened, what you learned. You couldn’t have got this far without becoming better. Whenever your viva is, remember that throughout your PhD you have become more than what you were at the start.
More talented. More skilled. More knowledgeable. More accomplished.
All the stretches along the way have helped you become who you are today. Remember that and be confident for the viva.