1. Leave your ego at the door. As matter of fact, divorce your ego. Trust me. Your work will THANK YOU.
2. Break it up into parts. Don't count the red marks all at once. Unless you're a masochist.
3. Whenever you feel like it is too much, always allow yourself to focus more on the good part of the editing (example in the picture).
4. Author is the creator. Author is the boss. We make the final decision. But we are not perfect; we make mistakes. That's why we need help from others to edit our work. I don't believe in self-editing. It's not a matter of "can or cannot", I'm pretty sure a lot of authors are capable of editing their own work. I just don't recommend it.
5. Always, and I mean always, treat yourself to something awesome and fancy once you've passed (and survived) the whole editing process! You earn it. Big time.
Picture is taken from "I WON'T BREAK" first draft, a My Lea sequel by e. mellyberry.