Having a brain fog, how do I stay focused on the exam and practice day?

Hi everyone, I’m preparing for the PTE Academic exam in 3 days. I keep getting brain fog, feel tired, and my focus is slipping during practice tests. I’ve been sleeping okay, but when I study or do mocks, my mind goes blank or I jumble words in speaking.

Has anyone experienced this? What strategies, foods, or routines helped you stay alert and focused right before the test?