Struggling to revise

My GCSEs start on Monday and I literally can't revise. I cannot concentrate on anything because I'm too stressed and because of that I'm hardly getting anything done. I generally get high grades (mostly 8s and 9s) in mocks and stuff, but I'm terrified that the exams are going to be way harder than mocks and that everyone in year 11 is really smart this year so grade boundaries will be really high.

If anyone has any tips, I need them urgently