Most people (including many professional writers) consider writing difficult. In fact, it's probably the most frighteningly difficult thing most people do in their professional lives, second only to public speaking.
Part of the reason for this is that people acknowledge, I think, on a gut level, that writing is a form of artistic expression, and yet most of us (for whatever reason) are convinced we're not capable of "art." No one is asking you to create art, though, so why impose that expectation on yourself, unless you're writing a sonnet?
Much of the fear of writing comes down to the fear of producing embarrassing crap. But here's something you should always bear in mind. Everything you've ever read, in print, started out as something crappier than what you ended up reading. You've only ever known the glossy polish of the final product. You didn't see the crappy rust-covered underbelly of what came before.
So don't hold yourself to a "final output" standard when you sit down to write. Your first effort may well be crap. But then, so was everyone else's first effort. You just didn't see it.
Always give yourself permission to produce crap. If you don't, you might never get past the first sentence.
Rumination is important preparation for writing, so always give yourself as much time as you can to think about your subject before sitting down to write. Break your topic down mentally into the simplest possible bits. Think each bit through on its own. What would you say in 25 words or less about each bit? Take notes if that helps.
I rarely sit down to write on a topic that's less than 70% to 80% thought-out in advance. (I count something as "thought out" if I am 90% confident that I understand my own unique take on the subject and can write about it in a way that will fool 90% of readers into thinking I know what I'm talking about.) I know I can count on the writing process itself (which is iterative, reentrant, non-linear, and thus organic) to help fill in the missing 20% to 30% of thought-outness, but I never count on (say) 90% of what I'm going to write just "coming to me" as I write. Only 20% or so will "come to me," and most of that in the editing pass, after the crappy first draft is laid down.
I like to think of putting fingertips to keyboard as the last step in a long chain of preparatory actions (research plus rumination). Captured keystrokes are just a static artifact of the dynamic thinking process that went before.
One of the best insights I can give you is that simple thoughts are easier to write down than complex thoughts, so anything you can do to de-complexify your thinking will have a huge payoff when it comes time to write. Stuff that's arduous to write is usually arduous to read. How do you make the arduous easy? Simplify.
If you find yourself completely choked up when you sit down to write, it's either because you're afraid to produce crap, or because your thinking on the subject matter is still muddled. If you're not under a deadline, take more time to think the subject through, except in simpler terms. (You can always make simple stuff complicated later, although I don't recommend it.) If you're under a deadline, sit down and quickly write a bunch of absurdly short sentences on the topic in question, phrasing everything in unacceptably oversimplified terms. I guarantee that if you quickly fill a page with laughably simplistic one-liner statements about a subject, the Fussmaster General inside you will be eager to jump at the chance to cross all that crap out and do a more meaningful job of expressing the same thoughts. In other words, you'll be ready to write "in anger."
One more point and then I'll shut up.
Techniques exist for simplifying the expression of ideas. Most of them revolve around semantic clarity and micro-syntax. Just by avoiding certain types of words (for example, gerunds, which are verbs pretending to be nouns) you can force yourself to write in a simpler, clearer manner. The neat thing is, simple writing always comes out faster (and reads better) than turgid-but-logically-complete writing. The simpler you write, the easier the process, for you and the reader. I'll be talking about some of my favorite techniques in this regard over the next few days.