Nothing is more frightening to a writer than a blank page. When you sit down to begin an essay, and as you go along, you can often feel anxieties that cause the writing process to be more painful or difficult than it needs to be. As a private writing tutor in Cambridge I've seen a variety of difficulties, but for the most part they all boil down to two fears: fear of not having enough to say, and fear of having too much to say.
The key to alleviating both of these fears, and thus moving confidently and competently through your writing project, is careful planning before you start writing.
Writers who fear having too little to say start out at the beginning of an essay and can't imagine how they are going to fill three pages, or five pages, or ten, or however long the assignment is. As they go along they either move very briefly through their ideas and end up short of the proper length, or they get nervous and try to fill out the space with details that are unconnected to the point they are trying to make.
On the other hand, writers who have too much to say feel constrained by the limits of the assignment. They have points they want to make, but every time they begin one they can see its implications branching off in ten different directions, like a wikipedia page full of hyperlinks. They feel that if they don't expand on all those related issues they won't be able to say anything sensible at all.
Any academic tutor will tell you that in both cases, the key is to brainstorm, and to have a clear idea of what you want to discuss in the essay before you start writing.
Experienced writers do this in their heads automatically, but if you are a beginner you will need to write it out on a piece of paper. An essay writing tutor can be very helpful with this part of the process, and as you develop your written study skills. The task is slightly different depending on which sort of problem if you have. If you are worried you won't have enough to say, then write down your ideas, or the points you want to make, and look carefully at the list. Think about what you are leaving out, or what questions you are leaving unanswered. Slow down and dig deeper into the ideas on your list to come up with a few more ideas that are closely related to the topic at hand.
If you are worried you will have too much to say, then make a list of your ideas and start to streamline it.
Try to see which ideas can be dropped, or assimilated into other points you will make. Let go a little bit of the worry that there will be unexplored questions in the essay. In an essay of just a few pages, you cannot discuss everything related to the topic at hand. Your job is just to come up with a basic idea, and then say what needs to be said in order to convey that idea to another person and persuade them that it's correct. The shorter the essay is, the more strict you need to be. Anything unrelated to your main point, or to the task of getting another person to understand it, needs to be dropped.