If you’re concerned about remembering certain things at your viva then it’s alright to use prompts. Highlight keywords on thesis pages, mark the beginnings of key sections and prepare summaries before the viva to help bring your thoughts together.
You’re not expected to be perfect, but you can help your confidence by finding useful prompts in your preparation.
You can directly prompt your confidence for the viva too. Consider what helps you to feel better and feel confident. It could be something you wear. A small ritual or item could help. Listening to a song or playlist could give a boost.
Prompt your confidence by whatever means help you so that you don’t forget that you are good enough.