Competent, Capable, Confident
If you can demonstrate competence in the various skills and processes that are expected of a researcher like you then you’ll be more capable overall.
If you can recognise in yourself that you are a capable researcher – because of what you can do, what you know and how you think – then you’ll feel more confident for your viva.
Feeling competent is only part of feeling capable; feeling capable is only part of feeling confident. Cover the basics but also think about what else you can do to build up all of these feelings as you finish your PhD and get ready for your viva – and the future beyond your doctorate.