It's actually really common to want to tell your editor how terrible your book is, or to apologize for it, when you hand it in. I think it's that by the time you finish a story you've been working on really hard, especially anything that's long and complicated, you've lost the ability to evaluate it. I have too much memory of what crap parts of it were before I fixed them and how long it took to figure out. I'm looking at scenes and seeing all the past versions of them and all the work it took to get there, while the editor is only seeing the most recent version, without all the baggage, and can see it only needs minor tweaks.