Editing work with seriously poor grammar is so frustrating. No wonder professional editors throw away submissions received in such states, without even reading them! Edit your work carefully and you will have more chance of success. Skimp on the (yes tedious) writerly necessities and, even if your work is excellent, it could end up unread.
1. Read your work aloud
2. Take a break as being too close to a piece means you will miss the mistakes. Go back and re-read it after a good break away, as if you are reading it for the first time.
3. Give it to someone else to read and if they struggle with it, edit it again