Diva,
I have this problem every couple of weeks or so when I have tests but I have gotten alot better about it and rarely freak out during exams. The visualizing is a great technique to use and I also tell myself that I will be done within a half an hour or so and then I can leave. This is sort of like a reward for me because I no longer have to sit through a two hour class, I get to take an exam and then leave therefore being anxious for a short time while taking an exam is way better than struggling two hours in a class. I just look at it as more of a reward now instead of looking at it as what if I freak out, fail, panic, etc. Also, you are studying and preparing yourself now so you shouldn't worry about failing. If you know the material you will do well just do some breathing exercises if you get anxious. I flip to the back of the exam and work from back to front when I get really anxious during an exam(not sure why but it helps me to be less anxious for some reason). Well, I hope this helps you. I am sure you will do fine.
~Isabella~