Writing, despite what you might think, is not easy.
It's straightforward enough to blather on and kind of get your point across - throw enough words at it and your reader will somehow get the point - but expressing yourself concisely and efficiently takes time.
This is how Massachusetts Institute of Technology neuroscientist Susan Hockfield describes writing:

"It's the painful process of transforming three-dimensional, parallel-processed experience into two-dimensional, linear narrative...
It's worse than squaring a circle, it's squaring a sphere.
"

Sounds tricky! You can sometimes get paralysed thinking about it.
What you want to say makes sense in your head, but how to put it into words? Don't worry.
It's okay to be bad.
In fact, bad is good! That's why we call them first drafts.
It's much easier to edit something than it is to compose from scratch, so the next time you find yourself staring into space, hoping for inspiration and dreaming of more pleasant things (Halle Berry, Bird's Trifle, Dennis Bergkamp's goal against Newcastle...
whatever), get something written down.
You'll be amazed at how much easier the job will be once you have text to look at and edit.
And remember: it really, really doesn't have to be perfect first time.
It can be incomprehensible.
A different language.
But no one ever has to know except you.
And me, of course, but I promise not to tell.
So get writing.
Doesn't have to be good, doesn't have to be usable.
Think of it as the Base Camp littered with used crisp packets and baked bean tins in your journey up the mountain.