I’ve lost track of the number of books I’ve read where editing was clearly not considered to be an important thing before the work in question was sent on to the publisher.
You can’t just rely on yourself to make sure that things flow properly and that correct words have been used in what you’ve written. It’s entirely too easy to simply skim right over an issue and keep going. We’re sort of geared for it, really, if you think about it.
To help you to put out the best version of your work, we’ve got listings of professional editors here who would be happy to go over your work and let you know if there are things that need to be changed/adjusted.
That said, if you are bound and determined to do it all yourself, for whatever reason and you’re not going to send your work off to a professional editor (or even a grammatically gifted beta reader who loves you enough to do it for you), then at least take the time to read your story OUT LOUD to yourself. You’d be surprised at how many issues you can discover just by doing that.