We all make little mistakes. It seems the longer and the more you write the more mistakes you make – and take for granted. I think we just get used to thinking we know what we are doing.
Being your own editor can only get you so far. Every now and then have someone else look over something you have written. Get them to spot check your grammar, your over use of any certain word(s), your punctuation and spelling.
Of course, pick a day when you are able to listen to their critique. You can’t ask for help and then argue with them or defend yourself as if you have just been personally attacked. When you ask for help accept it graciously. You can be sure you will need help again.
Make note of everything they tell you and keep it all in mind when you write again. You might notice how right they are once it has been pointed out to you.