If you are thinking about a piece of writing as incredibly important and having to be absolutely right it's very difficult indeed to write it.
The best thing is to think that it doesn't have to be right first time. It's easier to revise something than to write it. This is what I would do (and I suffer very badly from writer's block so I know whereof I speak):
Give yourself a time when you can get up and do something else (I invariably find the most productive time is ten minutes before time's up) so you are not sitting there miserably all day.
Try this exercise. Using a pen and paper, write for five minutes. You can write anything, utter nonsense (eg why i am doing this bloody exercise), the point is to keep writing WITHOUT STOPPING for five whole minutes. I usually find that by the end of the time I have started whatever it is I want to write.
Try and write as freely as you can without stopping to correct yourself, being grammatical and so on. This is just a first draft. The main thing is to get the gist of your thoughts down. Write and write as long as you can (or until your time is up). When you've finished, put it away, go and do something else for a while.
Then, come back when you have had some time not thinking about it (this is important because quite often your brain will work on it unconsciously while you are doing something else).
Now you can work on revising it. Probably you will change it a lot, but changing something that's already there is easier work than writing it in the first place. You may need to revise a couple of times having got someone else to read it in the meantime but you will be on the way to having it written and finished.
Hope that helps!