Even native speakers find it difficult to edit their own work, as your brain often sees what you were planning to write rather than what you actually wrote. A great tip for people who do need to edit themselves is to leave your work for at least two days and come back to it to do the final edit. You will be amazed how many mistakes you can now find. If you have spent those days doing any of the things mentioned above to improve your editing skills, you will also be able to pick out mistakes that you didn't know about when you originally wrote that piece.