You don’t have to carry everything around in your brain for the viva. You can make notes. You can make plans. You can clear out the clutter and make sense of your ideas. When you start to prepare, write down a list of all the things you think you’ll do in preparation, then organise them.
Underline typos in your thesis as you find them (or make a list) so you don’t have to remember them. Add useful notes in the margins to help you. Write lists of questions you could ask your examiners. Summarise anything valuable or anything difficult about your work that you can think of.
You don’t have to remember everything. Lists, notes and summaries can reduce the burden on your brain for more useful thinking!