Extreme nervousness during exam period
Nervousness in the face of uncertainty is normal. Ask any astronaut sitting on the launch pad. What you experience is pretty common. Overconfidence can also be a hinderance. 

You seem to have good study habits ("weeks in advance"). But note that cramming the night before an exam is counterproductive for nearly everyone. If you are tired, so is your mind. And it will be focused only on what you studied last, rather than having a more general and nuanced sense. 

One "trick" is to do something active in the last couple of hours before a big exam. Anything to get your blood flowing and let your mind rest. Even deep breathing as a calming exercise has benefits. 

I also have a bit of a mantra that I use at the start of an important activity, such as an exam. "Some things are beyond my control. I can only do my best."