When you finish writing the first draft of your manuscript, there’s a damn good chance that your novel is NOT done. In fact, if you expect your novel to be a work of permanent perfection, it probably never will be “done.”
Think of yourself more like a parent who will inevitably still see a grown child as a work in progress. Ten years later, after professionally editing and publishing your novel, you’ll wake in the middle of the night with some new idea for the book. “Ahh,” you’ll think. “How could I have missed that? It would’ve made a huge difference! But oh well. No going back now.”
Sure, you could just wait ten years to publish your novel, but who’s to say that after twenty years you won’t have the same “how could I have missed it” experience? There comes a time when “good enough” is good enough. So how do you know when you’ve reached that point?