We all do it. We write a book we love. It's wonderful. Its characters made us laugh. They made us cry. They made us fall in love. "That book is my baby," we say.
Until that manuscript goes into a drawer for a month while you let the ink dry, and the objectivity comes back.
This is what you must do as a writer, to distance yourself from the books you've written. Only after you haven't seen the work for a while, can you read it with fresh eyes. And when you come back and hate the words you've put on the page, and you're ready to burn it, and you think you should never torture the world with your prose again ... then you're ready to revise.
Being critical of your work is not a bad thing. In fact, it will help you get the thick skin you need to be an author. Your worst critic--you--is about to become your best ally on the road to publication.
First and foremost, you'll need to look at the mechanics of your writing. Are the sentences structured properly? Are you using poor grammar only where it's done on purpose (There are such instances!)? Is your book loaded with adverbs? Too much telling and not enough showing? Remember: Unless you fix the mechanics, your reader won't be able to see past clunky writing to appreciate the great plot and characters of your book.
Next, look at characterization, POV, and flow of your storyline. Is everything consistent? Do a character's actions and reactions make sense for his/her worldview? Do the scenes flow nicely? Are some too short and others too long? Do you get to spend enough time in each major character's head to fall in love with him/her? Could a scene be written better in someone else's POV? Cut scenes, move them, rewrite them. You can save the whole enchilada as a new file in your computer, so that you have the old version on hand, should you want to change something back.
Do not be afraid to slash and burn parts of your book. If the core of your story is strong and moving, it will survive the editing process! And if an agent or editor suggests a change to your manuscript, consider it carefully. Most of them are experienced in what sells, and they want you to get the best bang possible out of your manuscript. They aren't out to tear up your work, I promise.
That's your job. :)