Three challenges for the PhD candidate.
First Challenge: believing that you don’t need to have a photographic memory about everything connected with your research and your thesis. Your examiners don’t expect you to know every possible thing. They expect you to have done the work, done some prep and be a talented researcher in your field. You can show them this.
Second Challenge: recognising that the viva is not the hardest thing you will ever do, and not even the hardest thing you will do as part of your PhD. It’s a couple of hours, talking with experts about your work, and it matters, of course – but so does the thousands of hours of work that’s gone into producing your thesis.
Third Challenge: accepting that the viva, in most cases, is a reasonably enjoyable experience. Some vivas are tougher than others, with more difficult circumstances. It doesn’t follow that yours will be.
Notice that none of these challenges are faced in the viva exactly, but in the expectations for it. Rise to meet them and the viva itself will seem less worrying.