No author has perfect grammar. Not one. Me included. In fact, I would argue that if I were to publish something without have passed it by the keen eyes of Adrian and Amy, it would be nearly painful to read. This is from someone who has an English degree.
Here's the thing: I don't feel too bad about it. After editing my first novel and learning where my problems are, I have caught myself finding the errors in other authors' work that I didn't notice before, especially if they're the same ones I make. For example, I am prone to this sentence structure:
He rolled the office chair across the floor with his feet, reached back t turn the knob on the door.
That is not a good sentence. There should be an "and" instead of the comma, or reached should be reaching in order to keep the comma. So, how did I get into this habit? Well, a high school teacher once told me that I used compound sentence and the word "and" to much, and thus began a nearly ten year quest to avoid using the word "and". Turns out she was wrong and I was wrong for listening. Now that I know how often I did that, when I read some best selling author who has literally sold hundreds of millions of copies of their books and see that they do the same thing I did over and over and over again.
My other big thing is starting sentences with conjunction. I was almost constantly using "and" "but" "so" to start sentences.
Now that I've told you all about how bad of a grammarian I am, I'll get to the moral of the story....I got better. My second book was much better than my first, and I believe the third will be better than the second. I may be to the point where commas are my biggest consistent error, and I'm not sure anything will make that change.
The editing process is painful. It brings all your flaws to the forefront, and sometimes it feels overwhelming. Unfortunately, it's a valuable part of the process, and in my opinion, all authors need to find at least two good beta readers to find all those flaws. Because what's even worse than betas tearing apart your grammar is reviewers tearing apart your grammar after the book is published.