People sometimes ask me how they know they are improving if they are not getting feedback. My answer is to remind them that no one has to tell them they are getting into better shape if they are running three or four times a week; nor does anyone have to tell them they are getting better at playing piano with daily practice. The trick is to put yourself into the same frame of mind with your writing.
What that means is that you have establish conditions that allow you to experience your competence. When you set aside a time to run, and map out a route, you are defining the run in a way that (if you’ve done it right) let’s you relax and enjoy it at your own (if you will) pace. You decide how intense it’s going to be, and you therefore open yourself to the details of the experience. The same goes for practicing the piano. Since you are in control of the conditions, you can listen to what you’re doing. It should be clear to you what sounds good and what doesn’t, what is easy to play and what is hard.
With your writing, my advice is to decide the day before on a particular claim to support, elaborate or defend, at a particular time, in a single prose paragraph (of at least six sentences and at most 200 words). Deciding what you will say is like deciding what route you will run or what piece of music you will practice. Now that you know what you’re doing, you can focus on whether or not you are doing it well. Fixing the moment in time lets you concentrate fully until your time is up. Then you stop.
It’s difficult to explain this in a way that is as effective as simply having the experience. Try it. Try writing a predetermined paragraph for a predetermined amount of time. Whatever else you may think of that experience, it brings the act of writing, and your ability to carry it out, to the fore. That’s the entire point of doing it this way. It makes improvement palpable.