That’s not you. Whatever your misgivings, self-doubts or nerves: you could only have got through a PhD to submission and be preparing for your viva if you were good enough. That’s the only way.
If you have a specific concern about your research, talk with your supervisor or a trusted colleague and explore why you’re concerned. If you’re concerned about the process of the viva then find out more, learn about regulations and general expectations to get a full picture. If you’re not sure if you’re ready then learn what it takes to be ready (it doesn’t take much).
If you’re nervous, you’re not missing something. You’re right where you’re supposed to be. Feeling nervous is a way of recognising that something is important, not that something is wrong. You’re not fake, you’re not deficient: you’re human. Do what you can to build your confidence. Count your achievements. Reflect on your talents and how they’ve grown through the PhD. Don’t look for things that could be better, look for things that are already good enough.
You’ve got this far because you are good enough. Keep going.