I don’t really know what will specifically help you, but can I ask what sort of revision you’re doing? Because what might help, if you’re not doing it already, is doing past papers, marking them and comparing what you got to grade boundaries. Since you’re revising a lot I imagine you’ll be doing really well and will get the grade you want on the practice papers, so seeing that and reminding yourself that you’ve proven you can do it and already got the right grade on the past papers might help?
Aside from that, are you thinking negatively about any parts of the exams in particular or is it more general. If there’s anything specific, let me know and I can try and help you with those specifically.