A simple piece of viva prep and confidence building: make a list of as many good things about your research and thesis as you can think of. Add anything about your development too, what knowledge or skills you’ve built up over the course of your PhD years.
It’s fine to list items, but even more powerful if you go back and add details as to why these things are good. Why is that result or piece of research good? What did reading that paper allow you to do? How does a skill help you?
A PhD can be hard for many reasons. There’s a lot of good there too.
Find the good, and use that to help you feel ready for your viva.