This probably should have been the first of the self-editing tips, but it’s all good.
Before you start self-editing your finished manuscript, give yourself a cooling off period. Put the book down and step away. Don’t look at it for a day or two or three… four if you can manage it. Five if you’re really strong.
While you’re writing, you are caught up in the high of the story climax and the conclusion. It’s the writer’s equivalent of a runner’s high. You need to come down off that high before you can self-edit properly.
You can’t look at your story with a critical eye while you’re still occupying your characters’ world.
Give yourself a day or two of earned recuperation. Relax and bask in the fact that you finished a book. Once you’ve relaxed and everything is level again, open up your story and make it better.