Here are some things I've learned to do when working on revisions:
Pretend like someone else wrote it.
If you can detatch from your manuscript and look at it with the cool eyes of a stranger, you'll be more able to do what needs to be done. Don't let yourself remember the joy and pain of creating that particular character or chapter. You're the calm, cool stranger reading this book for the first time. Even better: you're like the scary mercenary that gets hired in a movie when a hit goes wrong, like Harvey Keitel in Pulp Fiction, the one that can fix anything. Yeah, that's you, the scary mercenary who can fix anything -- and isn't going to get all weepy and mopey about it.
Be willing to change anything. Anything.
Don't cling. You're not married to that first draft. That first draft is more like a first date, and you get to go on lots more dates before you decide whether you're going to marry it. (I realize now this is not a good metaphor. I am not advocating you try to fix the people you date and turn them into your perfect mate. It doesn't work with people. But it does work with manuscripts. Yay!)
Remember: your book can get better. And better. And better.
It's unlikely your first idea is the best possible idea for any given situation. Revisions are an opportunity to take a raw thing and make it SO. MUCH. BETTER. Don't just fiddle around with the sentences as you've written them. Cast your mind out wide for any new ideas that might make your story more exciting, more beautiful, more whatever. Be willing to rewrite A LOT, to cut whole chapters and write whole new chapters.
"It is discouraging to try to penetrate a mind like yours. You ought to get it out and dance on it. That would take some of the rigidity out of it." -- Mark Twain
Love that. Dance on your mind. Go forth and revise.
Oh yeah, and:
"It is never too late -- in fiction or in life -- to revise." -- Nancy Thayer