It’s right to reflect on the significant original contribution that your research makes to your field as part of your viva preparation. It helps to consider how you can share that. If your examiners asked you to dig deeper, what would you say? What would you focus on?
Making notes, writing summaries and talking can all help to make that easier in the viva.
While it’s right to focus on the contribution in your research and thesis, it’s also important to invest time reflecting on yourself: what is the significant and original contribution you have made to your own development while working for your PhD?
What have you learned? What do you know now? What can you do now that you couldn’t before?
You need a good thesis to pass your viva. You also need to be sure that you are a good candidate. Reflect on the contributions you have made to both over the course of your PhD.