The success you’ll find in your viva is cumulative. It’s built on top of everything you’ve done over the last few years.
It’s not an all or nothing, once-in-a-lifetime event. You’re there because of every little thing (and quite a few big things) that you’ve done well, got right and been amazing at over the course of your PhD.
Find your confidence for your viva in all of your other successes that have come before.