Self-editing is best done as a multi-stage process. Don’t expect to edit everything at once and be done with it. There are a great deal of articles written about this topic and here is a link roll of them!
There are also an increasing amount of programs to help writers with their editing as well, beyond the simplistic red-lines of Word Documents.
Editor for Windows (one-time $)
AutoCrit (subscription $)
ProWritingAid (subscription $)
SmartEdit (license $)
Of course, editing software only suggests what you might overlook due to being tired or too close to the work or obvious grammar errors. Don’t automatically fix everything that a program suggests, think about whether it actually fits what you’re trying to accomplish with the writing. Basically, don’t expect to use any software as a crutch to avoid actual work when it comes to self-editing.
One of my favorite tricks when it comes to self-editing; start at the last word and work backwards with each sentence. Hold a piece of paper’s edge to distinguish which line you’re on. Be brutal in notes and decision-making.
Be honest about what works and doesn’t work, if you can identify an issue, then you should be able to problem-solve how to fix it. The answer is in there somewhere, otherwise you wouldn’t be able to identify the issue at all.