Beginning, Middle & End
You could be enthusiastic but untalented at the start of a PhD, and unsure but hardworking in the middle.
The only explanation for getting to the end is that you’ve done the work and done it well. It’s not an accident you’ve made it this far: you’ve done something that’s valuable, and you can only do it by being good at what you do.
The viva’s not a formality, it’s just one more challenge – a challenge you are fully capable of making a success.