"Writing is rewriting." –Donald Murray
Murray's quote is one that's commonly referenced (sometimes stated as "writing is revising" or some other variation). However you've heard them said, these are three simple words of hard truth.
While everyone has their own process, the revision phase of writing is typically a longer journey than the initial drafting.
My favorite way to describe it is to imagine a potter with a slab of clay. They'll get the initial shape quickly, but it takes a lot more time to refine that clay into something recognizable, to get the curves they want, to smooth it out, etc.
When you revisit your first (or second or third) draft, remember that you’re in that shaping phase. It can take a while to get a book to its final form, and that’s okay.
Give yourself permission to make a mess. The shape of your manuscript will reveal itself.