To me, there's more work put into editing than writing a book.
When editing you correct grammar, add sentences and scenes to make your book make more sense, take scenes out, rewrite things, fix the format of the book, etc., etc...So much effort is put into editing a book. And you can't do it just once.
I just finished editing the last novel I wrote and I'm about to start faze-two of editing it. I've changed the way I edit. I edited The Darkest Light almost four times and there are still a lot of mistakes. When I edited it, I didn't take out enough scenes either. There are a lot of things I now realize I should have done differently with it, but I published it and there's nothing I can really do about it now. I've moved on. But now I know for next time.
Here's how I'm going to start editing, I think.
Phase 1: correct grammar, add a sentence here and there, rewrite things here and there.
Phase 2: decide how to change format, and add scenes/sentences to make book better.
This, I'm hoping, will make editing go faster. Maybe I'll have to edit it one more time because I'll be adding things, but I won't worry about that unless I decide to self-publish this one too. If I can edit like this, I won't have to re-read the same book over and over and over again and get bored with it, and there shouldn't be as many mistakes when, and if, I edit it a third time.
So, yeah, that's how I'm going to start editing my novels.
You should always edit your book. Nothing about your book is perfect. Nothing. It always needs to be fixed. There are always things about your book that will need to be changed/corrected. I find mistakes in books that I read (that are traditionally published) all the time, and they have professionals checking their books.
So you should always edit your book multiple times before submitting it to a literary agent, editor, or publish it yourself. Editing is really important. If you don't edit, agents and such won't take you as seriously because you've basically half-done your book. If you don't edit it, you haven't really finished the book.
Happy writing! <3