No matter how much or how little you study one thing is crucial for you to realize; you are being tested by an examination so you must practice answering examination questions. How you approach these practice questions will greatly affect your exam success. Here's what you have to do. Never answer questions with the text book or study notes in front of you. It is essential that you practice your exam questions under the same conditions you will face in the exam!
Even if you promise yourself not to look, a brief glance unknown to yourself will help you answer a part of a question and lead to believe that you know it when you don't! When you find yourself hitting a brick wall in an exam on material you thought you knew, this can be one of the causes. Even the psychological comfort of having your study notes nearby is something you won't have in the exam and once you find yourself without them you will feel the pressure.
Time yourself. Write down the time at the start of each question and at the end. Don't worry if you go over the allocated time, you will improve with practice. But do keep an eye on how long you are taking. If you are consistently over the time, then either you are writing too much or you do not know your content well enough and you are taking too long to think about it.
Usually if you have practiced a lot and are still taking too long, you do know your material but are writing too much. Pay more attention to the wording of your questions, and the marks awarded for each section, if this is available. This will help you gauge the depth required. Ask your teacher to correct some of your answers to see what you can safely leave out.
All the best...................