I am a a circuit designer and I have passed the phase of taking courses and learning. I am still learning stuff in math but purely out of interest. After my experience doing research in circuit design, one thing I figured out is that it is not super important to ace every course. If you understand the concepts, you will be able to do well later on as long as you are motivated. However, in this context...
I know what it feels like to freeze up. We had a lot of courses in signal processing and in each course, we were given very tough questions to solve that too in the final exams. I had no clue what to do. I used to shiver and my hands literally shook out of fear during the exams. Believe it or not, when I came out the exam hall, it used to take me just 5 minutes to get the solution.
I am sure calculus is different from signal processing or error control coding. Nevertheless, the professor can give tricky questions. The idea is to give tricky questions to differentiate the smart ones from the "not so smart" ones. Unfortunately, a lot of smart ones miss out simply because of a psyche factor.
I hope I am not digressing too much from what you need to know. Most of the advices given above cover what you must do in order to do well; however, I am concerned about one thing that you mentioned... freezing up. I have had a terrible time in the past with the psyche factor. EE department of IIT Madras used to make life of its students a living hell because they wanted to make a clear distinction between the smart ones and the not so smart ones. In the process, they made the papers so tricky that almost 80% of our class had failed in at least one subject. I survived :). We had to take calculus in the math department and it was not easy either; however, the most we got psyched was in subjects related to communication engineering and analog circuit design. I froze like no one else and eventually, managed to get better.
Apart from working hard, please do work on becoming more confident. The exams are just one hour/3 hours in duration. The examiner might give you trick questions. Imagine that you are facing that moment when in your head, you will be thinking more about your grades than about solving the problem if you get psyched. My suggestion is to get AS MANY older papers as possible and solve them in a time constrained environment. Do your exercises and all that but on top of that, make sure that you solve as many older papers as possible.
THE ONLY THING THAT WILL HELP YOU IN SUCH SITUATIONS IS CONFIDENCE. I had to fight a huge battle with my diffidence especially when the seniors were telling me "this professor likes to fail the students" while what the prof was really doing is that he was trying to maintain a high standard in his class and maybe, slightly differentiate the smart ones from the others.
The bottom line is to work hard and understand concepts by solving a lot of problems. On top of that, I feel that the most important thing is to FEEL CONFIDENT. One of the things to acquire that is to solve yester years' question papers in a time constrained environment. Before you go to the exams, you must have known already deep within that "yes, I can solve a reasonably difficult paper in a time constrained environment even if it is a very tricky question paper". MAKE SURE THAT YOU ACQUIRE THAT CONFIDENCE.
Writing proofs is also an art. There are direct techniques but there are also proofs by contradiction or contrapositive methods. An advanced mathematician is quite familiar with those methods. The prof might just give a question requiring one such method and half the class will simply shake during the exams thinking "boy.. you must be a genius to solve this problem." If you have solved a variety of problems before and know these proof structures, then you might be able to do better knowing which method to apply under different situations. While solving problems, please give SPECIAL ATTENTION to proof structures. It will help you in other areas of math as well.Long story short, just practice solving as many problems as possible. If you are reading a book like Spivak, get the solution manual. Refer to it after you have tried the problem for like an hour or so. Make sure you completely understand the structure of proofs along with (obviously) the technical content.
Finally, solve problems from earlier years. Without the CONFIDENCE to go to an examination hall and sit there for just an hour or so to solve the easy ones as well as the tricky ones, you may not be able to give your best. The most unfortunate thing is that sensitive people (like me) tend to get bogged down simply because the situation seems to be really demanding. It is true that the situation is demanding. The key is to KNOW and completely REALIZE that you can crack the dang paper completely.