The Shift
Viva prep involves shifting the kind of work you were doing to submit your thesis to the kind of work that will help you be ready for your viva.
Viva prep operates at a different scale. Perhaps a few dozen hours are needed in total compared to the weeks and weeks of effort to get your thesis finished for submission. Viva prep requires focus on a few helpful activities while submission preparations need overall awareness that everything is done and nothing is missed. Viva prep benefits from a break to rest but submission can feel like there is no time to pause.
Submission is a challenge. The viva is a challenge. Both come in quick succession, relatively speaking, but both come after years of experience of someone rising to meet a challenge. If you work through to submission you can shift your attention and work through viva prep.
And then succeed one more time in the viva.