I know what you mean. I'm taking A2s at the moment which are much easier but sometimes I get really frightened that I'm going to fail and mess up my whole future. I was fine last year and didn't really notice the pressure but now I've been having nightmares about failing/bizarre things happening in the exam hall etc.
And sometimes I get scared to the extent that I mess up due to my fear rather than lack of knowledge or ability. I took an important exam recently where I couldn't concentrate at all. Every few minutes I would stop and look around the room and think, 'oh god, if i dont do any more questions ill only have got thirty percent and ill have screwed up my life'. needless to say, this obsessing made me do far worse in the exam than i would have done otherwise.
i think that everyone has to get over this fear. firstly understand that youre not alone by any means. there is a huge amount of pressure on us nowadays that there wasnt in the past. accept that there is a lot of pressure, accept that you are going to feel stressed and don't beat yourself up for getting worried.
secondly remind yourself about how much you've already achieved and all the times you havent screwed up. why should this time be any different?
and thirdly concentrate on the work itself. not the grades, not the exams but the actual work. if you prepare well now, even if you panic in the exam, chances are that you will be able to overcome your nerves and get a good mark. draw up a good revision timetable. divide it into days. and stick to it. think 'today im going to do chapter 1.' that's it. no need to worry about chapter 2. no need to think 'what if i dont finish in time'. just do the work for the day. remind yourself that you enjoy the subject, after all you chose it for uni.
if you feel super stressed out, take a short break, say ten minutes, close your eyes, listen to some music, then get back to work.
another idea someone told me is to write down all your worries at the end of every day. say from 9 to 9.15 every day is your 'worry time'. if you think of anything else during the rest of the day, push it to the back of your mind. you can think about it in your worry time. also helps to write it down and get it into perspective.
why do you think youre going to fail? do you find the work difficult? then ask a professor or fellow student for help. are you behind on revision? then get on with your revision. there's still time. are you bad at taking exams? obviously not, since you've got this far already.
hope this helps a bit
xx