Writing is difficult for two reasons: (i) writing requires thought and thinking is difficult, and (ii) the physical act of putting thoughts into words is difficult. There’s not much you can do about (i), but there’s a straightforward solution to (ii) — namely, writing. The more you write, the easier the physical process becomes. I recommend that you start practicing with email. Instead of just dashing off an email, write it. Make sure that it expresses exactly what you mean to say, in a way that the reader will be sure to understand it.
Bonus quote from Lamport, and I will stop, I promise. (Although I can give several similar quotes from Dijkstra also.)
Math is nature's way of letting you know how sloppy your writing is; mathematical rigor has proved a solid foundation for other engineering disciplines — why not ours?
Lamport is a renowned distributed systems researcher, inventor of Paxos, and creator of LaTeX.