Do everything you can to improve your feeling of confidence for your viva. Look back at the story of your PhD and take apart all the key moments and stages to show how you have become a capable, talented researcher.
Examine your successes. Unpick the work you did, what happened as a result and what that has meant for your research and for you.
Explore your setbacks and failures. Why did things go wrong? What did you do to change your approach as a result? What did you learn from those experiences?
Estimate the amount of work you invested. How many days did you show up? How many hours? Realise that you stayed committed to your work, even when it was a challenge.
What other numbers could help you to see the scale of what you’ve done? The number of words written? Papers read? Meetings attended and talks given?
Find ways to remind yourself. Statistics help, but stories help more. Do things that help you to feel confident. Wear clothes that help you feel good. Your confidence is not only rooted in the pages of your thesis and what you did to write them.
Use every opportunity you can find to build up your confidence for the viva. There are many, many reasons to feel certain of your success. Do what you can to remind yourself of them as you get ready.