So, after a full week spent agonizing over the project, I came clean to the editor - told him that I wasn't feeling as though the manuscript was quite up to snuff and that I was sincerely sorry for potentially having wasted his time.
And, thankfully, he was very understanding and respected my decision to withdraw the manuscript from consideration. He even asked that I send the fully revised version to him directly, should I change my mind. And he asked to that I keep their press in mind for future works as well.
Now, to focus on completing a novel (that I won't retract after submitting) before I turn thirty. That gives me - gulp - six months.