In life there are problems you can do something practical about, and there are problems that can only concern you. Realising you can do nothing to change a situation or problem doesn’t make it go away, or give you some kind of special awareness of it, but it can free you to work on problems that could respond to action.
So, as your viva gets closer, make a list of problems that worry you: your thesis, your research, your knowledge, your talent, your examiners, the viva, everything.
Then for each of these things ask yourself if you can really do something about it. Is this a problem that can be resolved or improved?
If it is, what could you do? Then what will you do? Great! Make the situation better.
If it can’t be resolved or improved, if it just is, then what will you do?
You could obsess about it, you could keep exploring to find a solution, or you could try to find a way to cope with it. Maybe all you could do is know it’s an issue, but choose to focus on things you can improve. That’s not necessarily easy, but it might be for the best.
Take time to see what’s really going on for you around viva time. It’s not always easy to change how you feel, but you do get to decide how you act.