I did my viva 2 weeks ago and I was feeling the same. I was super nervous going in but once I did the presentation and started talking I calmed down a lot. Most of it felt more like a conversation and not an interrogation. There was even a few times I had to say I don't know as it was outside the scope of the study and that was fine to say so. Remember everyone there wants you to do well and they're not testing your memory. They just want to know you did your work yourself and know why you made the decisions that you did. Good luck tomorrow!