Good essays are usually not written in one go, rather they develop over time. Just like when facing the mammoth task of a Christmas meal, some of the trimmings might not be appreciated because there is just so much of it, and you end up with cramming stuff in leaving some things hard to digest. And things get left behind, like referencing your sources (the unloved Brussel sprouts of the academic world).
Approach your writing more like snacking – write often, even if it is only a little bit, write little, but regularly. This works best if you timetable this into your regular routine – make a habit of setting a little tim aside for your writing, and you will have enough content to edit in no time!
(I was introduced to the concept of Snack Writing by Nancy de Freitas)