I’ve written before about how nervousness is strongly correlated with important things. You have a choice: you get to pick which you give attention to: do you try to make yourself feel less nervous or try to do the important thing better?
There’s also a strong correlation between important things and wanting to pay special attention to those things. Pay attention to the celebration. Decide how you are going to mark it.
Plan the party, if you’re going to have one; figure out the reward, if that’s more your thing. Tell your friends, “Help me fix this in my mind as a big deal!”
Because it is a big deal. Passing the viva, finishing the PhD, is a very big deal.
Thinking again, perhaps it’s not that there is a correlation between the important thing and wanting to celebrate it. Perhaps the celebration is a sign it is an important thing?