You probably can’t commit your thesis to memory, getting every fine detail stored away somewhere.
But you could make lists of key places in your thesis where you can find useful information. You could make a couple of summary card prompts. You can highlight important sections with Post-it Notes. You could create a little index card with page numbers and helpful hints. You can do a lot to make your thesis clearer.
You don’t need to remember everything for the sake of the viva. Doing what you can to help your memory of the details will help your confidence.