Why did you start your PhD?
Why did you keep going?
Why did you make progress?
Why were you ready to submit when you did?
Why are you going to be ready for your viva?
You had your reasons that got you started in your research. While those might change as you keep going, you still have your whys as you head to the finish.
Circumstances for your viva might change, pressures might rise up that you were not expecting. Keep a hold of your whys – why you’re doing your PhD, the fundamentals – and you’ll get through.