I know this is easier said than done if you are an indie author on a limited budget. A decent editor will cost a lot, more than most hope to make out of their book. That leaves friends and family. The problem there is that they might just tell you how good it is, because they are on your side. What you really need is someone to tell you how terrible it is and where it needs more work.
I’m lucky a very good friend of mine is also a successful author and she has gone through my work three times so far picking it apart. It’s quite hard when you get the comments back. They normally start with something like well done it’s better than the last draft but most it is terrible. I’m being unfair she never calls it terrible, she gives great constructive advice.
What do you do if you don’t have a friend who is a successful author? Swapping work with another author might be a good idea but then you must give up your precious writing time to read and critiques theirs.
There is of course writing groups but then if it’s like mine you only read short passages, so you must apply all there, hopefully helpful, advice to the rest of the book which they haven’t read.
I’ve recently being pointed towards so redit groups that will critique your work, but you must as well. Again, it’s going to be parts for your book which you may not want on a public forum before publishing and you have to make time critiques others work.
I think the only answer is persuade your partners or best friend to go on editing courses, good luck with that.
How does everyone else get over this problem? Or do you just self-edit and hope for the best.