The Nth Test
The viva is the only exam of your PhD but it’s not the first test.
How many challenges have you faced and overcome?
The viva is just the next test. You’ve managed N-1 for some suitably large number N over the past few years. The viva is one more challenge, one more test, even if it is the first exam.
The viva is a particular challenge so find out what to expect and prepare – but remember that you have done more and can do more. You can overcome this Nth test you face.