Oh I know, well I don´t know exactly what you are getting at...anyway, I can guess what how you feel. So, perhaps you feel like you wading in some waters, like crossing a river with water up to the neck.
That is, you may be stunned by the sheer number of things to recall when writting, things like grammar, coherence, coesion, etc.
Maybe it´s a good idea to buy and use some book(s) designed exclusively to writting. Exploring Through Writting by ANN RAIMES, St. Martin Press New York
or Writting and Grammar - Communication in Action by Ruby Level
Handbook Edition, Prentice Hall press. could be a good idea or any other that aim to hone that skill in ESL students. I got these in second-hand bookstores, so you might find them even in such bookstores, even online, maybe.
Well, just an idea here, if those don´t help you maybe it be of service to others in the future.
And don´t forget to hone your other skills as well, don´t forget the listening, speaking etc.
I wish you good luck on your exam.