In the viva, as with the rest of your PhD and life in general, there are things you can control and things you can’t.
You can control what goes into your final thesis, but you can’t control what your examiners think of it.
You can control how much you know about your research and your field, but you can’t control what questions come up.
You can control what you do to feel confident, but you can’t get rid of nervousness completely.
You control the actions you take. Focus on what you can do to be prepared, not on things which are beyond your control.