That’s what your viva is likely to be. Possibly your first and last time even being in a situation like the viva!
Focus on the first: you can’t expect to be perfect. No-one is perfect the first time they do something. You can prepare, you can be confident, but know that you don’t have to be some unattainable ideal.
Focus on the last: you’re almost done. You’re nearly there. The hardest work is behind you. You can do this. You’ve done everything else that’s got you this far.