Motivation Lapse

I've resumed revisions of a mystery novel I wrote years ago. The manuscript needs extensive changes, primarily plot-related ones. I searched the Web looking for plotting tips and found myself on the site of a UK literary agency; their advice on plotting mysteries is solid, yet formulaic. Not quite the inspiration I was looking for. Actually, it's not inspiration I'm lacking, it's motivation. I'm frustrated because I've plotted myself into a corner and haven't found a viable way out. Revising a book-length manuscript is a daunting task, but the greatest challenge awaits—the submission process....