If you find it very hard and very slow, then you're actually doing it. Congratulate yourself.
I like what Maryn wrote. Give yourself permission to write some really bad stuff. There is no law -- federal, state, local, or natural -- against rewriting or throwing the crap out later.
John McPhee had an article in the New Yorker recently about writing and rewriting. I was very cheered by it, considering how good he is. Essentially, it boils down to this: the first draft takes forever and more pain than root canal surgery. He finds the second, third, and fourth drafts all much easier, and they go much faster.
Amen. Based on my own experience (well over six thousand paid published news articles, speeches, a couple of print reference books, four e-books, and also quite a bit of very difficult writing -- direct mail solicitations getting people to send money (it makes fiction seem easy) -- I have this thought about rewriting: either like it enough to do two or three or more rewrites, or find other work.
I am progressing at a snail's pace, I have everything plotted out, the characters pretty well developed, I know all the scenes that are supposed to happen, but in my mind it's so big and fabulous, but making it happen on the page is another thing entirely. I feel intimidated by my ideas, and how to make them manifest. I have 49,000 words so far.
What should I do?