In addition to all the spotless suggestions above, my advice is that you should clock the time of your daily compositions. It helps when you are in the actual test/exam.
Try to get as much vocabulary as possible of the area you are planning to take the test, say it´s about law? Then seek law stories, news, essays, studies, abstracts, Forums, etc. It´s about science? Technology? Then see Science (and tech) mags like Wired, Scientific American...for Economy The Economist and so on. Practice makes perfect.
Ah, save/keep your essays and proofread them, even after you have graded them (it´s when we fall in the temptation to toss them away). What you wrote today you might expand, polish, alter, correct, comment on, etc the next day.
How good to see the progress we have made, when we see past texts and we see how we improved our writing skills!
Don´t get me wrong, I am not saying to you that you have to store trash, it´s just for a while, a week, a month or so. If it´s in the computer and you have space, you could store it for more time, if you wish. Until you take the exam.http://www.cambridge.org/elt/blog/2014/ ... ocabulary/
Ah! Last but not the least. Answering your question, yes, I had that difficulty. Once I was taking an English course and believe me, I knew everything about the subjects, until the exam´s day! Then it always drew a blank! Thanks God the teacher knew better and gave me marks for participation.
I am letting you know that, because you are not alone, you are in good company.
Don´t worry, it´s anxiety that takes the most of us in times like that, now I think I am a little less anxious and I write a little better. As you see, just a little.