Feel your pain. I suffer from exam anxiety. Here are my suggestions below.
Short term speak to your GP. They might be to advise some medical advice.
Deep breathing techniques might be useful. If you get the in the practice now it will hold you in good stead for the exam.
Mindfulness training- develops strategies to help you manage. Might also be worthwhile getting a book the little CBT guide which might help you rewire your thoughts. Ultimately you won't have this issue with timed exams.
Might also need to hold off on the caffeine and ensure you are getting enough sleeps. Helps you cope.Posted from my mobile device