If 99.9% of PhD candidates succeed in defending their thesis, why would you be any different?
You’re not the exception, you’re exceptional: hard-working, talented, knowledgeable, determined and capable. You might be nervous or have doubts; explore what these really are and get help if you need it. You might need to know more about the viva. You could need encouragement from your supervisors or friends. You could need help to get ready.
If approximately one in one thousand candidates fail at the viva, why would it be you?