how can I get over the dread that I may not get my ideal exam results?

in my previous mock exams, I got a grade 5 (C) consecutively in maths

i'm now sitting the real exams and i've challenged myself to get a grade 7 (A), it's still in reach after doing 2 papers out of the 3 but i just feel like i was over optimistic

i can't get over the fact that maybe it was a bit of a reach, and that I might have to just settle on a grade 6 (B)