I've just completed the first read-though of my second novel. I can say this with certainty: It's in a great deal better shape than the first novel was at this point.
I know it's not complete crap. I can see when writing is garbage and I can fix it. Crap is easy.
What I fear is mediocrity. That the book is just OK. That it won't live up to the first book and the publishing industry won't laugh when it arrives on their laps, but shake their heads and say something akin to "I just didn’t' connect with it." (I take that for code as "it's not terrible, but I have no clue how to fix this or sell it.")
And that feeling sucks. Just ask this poor kitteh:
I think many of us writers have it, and we don’t talk about it because it's so damned uncomfortable. When we stumble onto our little gems and smile, laugh, or cry at a passage, we think it must be over the top in some way or that we haven't conveyed the vivid pictures from our head onto the verbal canvas. I think that feeling results in a lot of murdered darlings. Some who met their gristly end for no good reason.
For that reason I *highly* suggest saving new copies of your document for each new draft. You may realize that you have wielded the purple pen of fury in haste and need to perform CPR on a sentence, paragraph, scene, or even a chapter that landed on the cutting room floor.
The trick is to let go of the fear. Your early drafts will likely be "just OK". And that's…fine. Take the time to polish and revise and seek out solid beta readers. Take the advice that works and with each new project take that next step (or two or three) from OK to great.
From book to book, You may take a few backward steps too, if you write long enough. Look at Stephen King, for a prime example. If you rank his many novels, they don’t' get incrementally "better" each time. Some are better, some worse and the chronology of when he wrote the book doesn't much matter. Carrie, his first book, rates high on many lists. The Stand is one of his very finest and he's written many books since.
And that, my friends, is OK.
I'm sure many prolific authors are disappointed when their current book doesn't perform as well as their past works. That's what gives them the drive to write the next.