To build confidence for your viva, write down three successes from your PhD journey. Think back over the years of work and find three things that have been good: maybe things you did well, results that didn’t exist before or ideas that you have developed and shared.
Find three successes and write a little about each, focussing on why they are a success and what you did to make them great. You don’t need to spend long to capture something good.
In fact, you could probably take ten minutes every day to write about three successes from your PhD journey. Between submission and the viva make a habit of looking back and finding three successes from your PhD: make a habit of building your confidence for the viva.