Confidence is not a destination. It’s not a permanent state you can arrive at, but a goal to be pursued.
You can’t flick a switch. You can’t simply hack or trick yourself.
But you can make a choice: what do you want to feel? What do you want to be?
Once you’ve made a choice, you have to act. For your viva, what would a confident version of you be like? If they’re not much different than where you are now, then you don’t have much work to do. If you feel that you could be more confident then choose to go for it. That doesn’t just make it so, but you’ll then see paths before you, steps you can take that lead you to your goal.