Am I overreacting about exams?

I recently did 3 exams. I am worrying about one of those exams in particular. Ever since I finished that exam I have been crying at least once every day (its been 3 days). There are still about 5 days until I get my results back. I am DEEPLY frightened about my mark. My mark in that subject is in the early 90's. I calculated that in order for me to increase or at least maintain that mark, I can make a max of 5 mistakes on the exam. I am soo scared that I have gone over 5 mistakes. I have actually begun to write down all of the questions I can remember from the exam just to make sure that I got them all right. So far, i believe I have gotten 3-4 wrong. Every time I think I got one wrong, I freeze up and I feel terrible. I know it sounds ridiculous but ever since I've been getting high marks in this class I have deeply wanted to maintain them, that's why I am going crazy now. The night I did that exam, I couldn't even go to sleep b/c I was so scared I got lower than 90%.

I believe that most of this craziness came from the trauma I experienced from after I did my science exam in first semester. After doing it I thought i did SO good, I thought i was going to get almost perfect. Everyday I would wake up feeling great, until exam review day. I ended up with a lousy 76%! It lowered my mark, and really did leave me shocked for days. Its like everything I built up, all the excitement came crashing down. I couldn't even look at what i did wrong, I just froze up. I remember before doing the exam I calcualted that I could make a max of 18 mistakes and still get an 80%, which was my target. I can't believe I managed to make more than 18 MISTAKES. I lost a lot of faith in myself after that. I remember the exam being quite easy and I was telling everyone how it was so easy. Ever since then I vowed I would never say that an exam was easy b/c it would bring my expectations too high and i would end up getting emotionally hurt.

On the day of exam review, I think I'm going to have to bring a pack of tissues b/c I think i'm going to cry before I get the exam, and if I get a mark lower than my expectations, I'm going to cry, and if I get a mark higher than my expectations, I'm still gonna cry-like crazy. I don't think i'm going to not cry at all on that day. & oh god, if somehow i manage to get lower than an 80% on that exam, I am not going to be able to handle myself. I'm probably going to go into a deep shock and faint-no joke. I want a high mark so badly. I don't want this to end up like my science exam, where I was shocked and feeling crummy for days and days.

Now, after all that, I was wondering if anyone else has ever felt anything sort of like that. Like that need to maintain a mark. It's driving me nuts. While everyone else in my grade is probably sitting down, relaxing, I'm freaking my **** out. :/