On the one hand, yes, the viva is your one and only opportunity. You have to do it, defend your thesis, engage with your examiners’ questions and discuss your work. You have to do it well enough to pass, and this is your chance to do it.
But on the other hand, it’s the latest opportunity you’ve had to do all these sorts of things. It’s not the first time you’ve talked about your work. It’s not the first time you’ve faced a challenge with your research. It’s not the first time you’ve had to really think about what you’re doing.
The viva is the latest challenge, for someone experienced at rising to meet challenges. It could be tough, it could be tricky, but it won’t be beyond you.
You’re the one and only person who could pass your viva.