What do you need the viva to be like for you to be happy?
Does it need to be short?
Do you need to know all of the answers?
Do you need your examiners to tell you what they think right away?
Do you need it to be at a certain time of day?
Do you need to set yourself up well on the day?
Do you need a particular kind of atmosphere in the room?
You can do something about some of these things, and nothing about others.
A better question might be: what can you do to be happy in your viva?
Answer the question, get some ideas, start some meaningful actions.