A friend of mine once told me the art of writing is rewriting. When he said this I had already rewritten my first book twice and I mean a totally overhaul not just a few sentences here and there. So the idea of rewriting it again and again was not something I was signing up for. It was perfect just the way it was.
To date I have rewritten part, if not all, of the first book at least seven or eight times. I've edited or had someone else edit it for me more than I can count. Writing is a process that never ends. If you set a project aside long enough and then come back to it you can always find ways to make it different. The question you have to ask yourself is although it makes it different does it really make it better?
I am sure when it's all said and done and available for people to read I will still look at it and think, "Maybe I should have done this or maybe I should have left that part out." But eventually I will have to let it go knowing I did the best job I could with the experience I had at the time.
So how do you know you've reached the point that changing anything will be for the sake of change and not necessarily for improving the book? Enlist people you trust and let them read it. If they like it then find a reading/critique group and let them read it. Make sure that the people who read it actually like to read and like the genre you write. If you let someone who only likes mystery and you've written a nonfiction you will probably get poor feedback.
Sounds pretty simple, right? It is if you don't care about the project you've been working on, but if you're like most writers you've put your blood, sweat and tears into it and love it like it's your own child. Taking criticism on something you love that much is hard. It's would be like putting your child on a stage and letting everyone tell you everything they don't like about them. It hurts and can make you angry if you let it and what usually ends up happening is you stop writing, you don't believe them even if it's true or you stop showing people your work.
All three of those options are fine if that is how you want to deal with it, but if you really want to become a better writer you have to be able to take the criticism and make it work for you. If your kid has an ugly haircut, change it. If they are wearing ugly shoes, change them. If your kid is just ugly consider plastic surgery or total makeover. (Remember that when I say kid I mean book or project. I am not promoting plastic surgery if someone says you have ugly children.)
If you want people to read your work and give you feedback you have to be able to take criticism. Some of this criticism will be constructive and that's the kind you're looking for in order to become a better writer, but some is going to be plain criticism. That kind of criticism is the hardest kind to take because they don't tell you how to make it better they just tear it apart and leave you hurting.
Remember you are going to get plenty of both and take it for what it is, someone's opinion. If their opinions are right change what needs to be changed. If they are wrong, forget about it and keep writing. Just make sure you know the difference.