It has been too many hours since I found your post so I probably can't help with this exam but for me a moderate amount of caffeine helps concentration. Studies show that it helps but some people are sensitive to it so amount varies. Also I would have caffeine before exam.

Don't fret over a course or two. I loved chemistry and did well in my first two classes but in my analytical chem I bogged down and went to the prof for help and he told me that I was bright and was great with the concepts of chemistry but I didn't have the math skills I needed. At that time I thought okay I don't have the math skills so I will do something else but looking back on it now I realize that if I had worked on developing my math skills and got a tutor I could have succeeded even though it would have taken me longer to achieve my goal than "normal". Incidentally I read that something like 60% of people require 5 years to get a 4 year degree. It is a marathon not a sprint.