I can somewhat relate. When I write essays, it takes me soo bloody long to write because I try to assert the most perfect sentences that I can conjure. (I'm not that good in english)... I find that my essays are incredibly static, and the essay does not hold well together in a holistic sense.
In exams, the first half is relaxed and I take my time... and then when I realise that I have wasted soo much time not really doing anything, I rush like crazy at the end. (I finish it... but its really bad)
Last year English exam, I wrote my last body paragraph + conclusion in less than 5 minutes D:
It looks like as if you have done a lot of practice already. So you should have a sound idea as to what you need to right. Just relax, and left the ideas just lift off your pen.
If you relax, you might also find that ideas just come to you. If you stress/ think too hard these ideas will just won't come. (like when you forget where you lose something.. then think really hard and you can't remember)