Like PL said bud. Just relax, seriously. Have a coffee, tea, whatever.. go for a walk around the block and start letting your mind wander on the subject. Get an idea of what you want to say (just a basic outline, not a word for word) and then write that out when you get back to your house.
DO NOT TURN YOUR NET ACCESS ON! Believe me, I know. I get the way you are as well when I have to write a report. IF you do need to check something online, do that. DONT 'just check your email' or 'just see what was posted on DS'. You will break your concentration then have to start again.
Good luck bud. Just write. Dont worry if you dont like it the first time.. read it through and change it. thats the advantage of computers.. you can change what you put on the pages.