If you’re concerned about saying the right thing at the viva – or worried about saying the wrong thing – then invest time before then in finding the words to share your research.
Write summaries to draw out key points. Review literature that has helped your work. Take time to rehearse with your supervisor or with friends.
The last part helps a lot. You can’t find the words by only worrying and thinking. You have to talk, because that’s what you have to do in the viva.