The rewrite progresses. People say: How can you bear to rewrite a whole book? Actually it's not rewriting, rather it's a shaking out and a holding up to the light. It's taking out the paint box and using stronger, deeper colours. It's deleting a mass of material that's not needed and including more of of what is. It's meeting each character afresh and wondering: What are you thinking now? What would you do now? Is this dialogue that you're having authentic?
To look back at writing which I thought as good as it possibly could be three months ago is a salutory experience. How can things be so different now? Partly it's having the benefit of a mentor to say: Just a minute.... you've really wasted that opportunity. Or: How can you expect the reader to sympathise with that character, after what s/he's just done.
The upside is that it's these few months of rewrite that transform the whole experience of writing a book from graft to something much more dynamic, and fun, even.