If you’re post-submission and trying to prepare for your viva but finding it hard then remember: you could not have got this far unless you were good.
Could you keep going for this long and this far if you were not talented?
Make a plan, ask for help, do a little each day, but remember: you did this. You got this far.
And if you got this far then you can get to the end of your PhD journey, and onto the next adventure.