I'd be glad to take a look at your essays and offer advice or suggestions in preparation for your upcoming exam. Having just taken an online peek at the writing requirements for the exam, I found the following:
The computerized GRE test is made up of three sections, verbal reasoning, quantitative reasoning, and analytical writing. The analytical writing section is always first on the exam and is composed of two different tasks. The first task is an issue task, in which the test-taker has a choice of two topics. The second task is an argument task, in which the test-taker must complete the one task given. Test-takers are given up to 45 minutes to complete the issue task and up to 30 minutes to complete the argument task.
The second "argument task" essay appears the tougher of the two types required by the exam. If you feel comfortable with posting your writing samples here, why don't you post a sample of one of your "argument task" essays back to this thread for review.
I'm sure that you'll find more than a few native speakers and English teachers -- besides myself -- who will be willing to offer you some practical constructive criticism here.