I heard an interview with Bill Evans, given in the last year of his life. (He only lived from 1929-1980 which means I've matched Bill Evans since I turn 51 today. I have the advantage of not being a drug user.) He said he had begun to listen to his earlier records, and that he could hear his playing objectively, as though it were another player's.
So one very helpful way of judging your own writing is to look back into the past. If you aren't as old as I am, even looking back a year or two can be very illuminating. The point is to have forgotten the thought processes that went into the sentences and to really see what they are saying.
Needless to say, you need to develop the capacity to judge writing that is not your own. If you can't do that, then you are unlikely to be able to do that with your own. Inexperienced writers tend to judge writing by looking for errors or shibboleths. What I am talking about is a very advanced and nuanced analysis of writing that recognizes positive virtues as well. Elegance is not simply the absence of inelagance.
If you can judge writing, then you can judge your own writing, given enough the lapse of enough time. Now see if you can judge what you wrote yesterday or an hour ago with a little more objectivity. Shorten the length of time, in other words. Now you have that inner editor built in to in your writing itself.
Why not just write a draft and have someone else be your editor? Well, you can still do that, but the better your draft is, the more the editor can put attention into higher-level concerns. Also, suppose you give your editor a draft and it comes back with comments like "choppy," "run-on sentences,""vague" all over it. Then the next time you submit something else to this editor, you should have taken some care with the length and rhythm of the sentences already. You aren't starting from zero with every draft, but internalizing the critique into your own self-editing.