I've always had a problem with timings. I'm predicted very high grades too, but my handwriting messes everything up, either I can't write fast enough or it becomes illegible. What I would do, is if you haven't already done so, make a revision timetable, and make a day where you do no revision at all, or half an hour of going through questions with someone else, and that is the maximum! Also, only revise on the mornings of weekends, don't push yourself all day unless you really have to, and try and make social arrangements at least every fortnight, even if it's someone coming to your house to revise, and then having an hour of free time.
As for the writing, just try and calm yourself down, I get really prone to stress around exam times, and I use this thing called Rescue Remedy which really works and you can get it at most pharmacies. Take it just before your exam or put it in your drink for the exam, and just try and stay calm, the more you worry, the worse your writing is going to be, and if you get stress writing (which is what I do and is where your writing goes all loopy and big) then stop yourself, count to 3, and then start again, that's possibly the best advice I can give you.