Don’t hold back. Say what you mean to say clearly and immediately. You can explain it later in your paper. Good writing isn’t cryptic, pompous, or inflated. It’s clear, honest and genuine.
Also, revise it until you’ve cut it down to its necessary elements. Re-read and revise as many times as it takes. No writing is finished until its author is dead, so don’t be afraid to tear your own work apart.
Finally, don’t stagnate in fear. All writers, from Composition students to Pulitzer Prize winners, are afraid that their writing will be crap. But when you sit down to write, do so. Don’t let fear prevent you from writing the fullest, most honest work you’re capable of.