And you don’t need to, in order to pass the viva.
You must have done something right to get to submission. You must be capable.
Feeling nervous or being anxious are general human conditions. If you feel them for your viva you’re either recognising the importance of the event, or have a specific concern. Both of these things can be addressed, perhaps not perfectly, but you can do something.
Rather than cross your fingers and fake your way through the viva, be honest with yourself. If you’re nervous, what are you nervous about? What can you do to genuinely build your confidence? If you have a specific concern, what is it? What can you do about it, or who could help you?
Trying to fake your way through a forced smile will hurt more than working to make the situation better. You can’t have faked it to get to submission. Don’t start now.
(also, literally: you can’t fake being as good as you are – you must actually be pretty great!)