There might be lots that you don’t know. If you tax yourself by thinking about all of the papers that you didn’t read, all the practical research you didn’t do (or couldn’t do) and all of the things you know that others know but you don’t then you could easily talk yourself into feeling bad about your thesis or your viva.
What you know, matters. You don’t know everything. You know enough. The things you know, the things you can do, have got you where you are. Not everything, but enough to get you to the viva and through the viva.
Not knowing something isn’t comfortable, but what you know can be enough to help you find confidence for your viva.