Essay writing gets hard for a few reasons:
1) Trying to write the 'perfect' essay or just worrying about it creates stress that blocks language flow.
2) Not having been properly taught how to write the type of essay in question.
3) Not actually following the steps taught.
For example, many students skip over doing the planning/outlining of their essay before beginning writing and this can do them in. Or they don't develop a thesis statement to begin with. If 1 is your problem, just spend some time brainstorming the different points you are going to write about. Don't worry so much about it all being the 'right' or the 'best' thing to say.