Put the finished and as yet, unpublished, work aside. Start something new. Let it take your mind over. After a suitable amount of time take a look at the other work...(this can be days, weeks or even months).
I just read something interesting in a book about writing: before starting to edit, read through the entire manuscript...you can jot a note down here and there but don't change anything. We need to see what we've got before hacking it up--THEN you can start the editing process. For me, the difficult thing has been staying away from the manuscript...I wake up in the night re-wrting sentences or even thinking about new plot twists...that's why NEW writing is so important at this stage of the game!