I have a great critique-partner team: four writers who read my stuff at various points in development and have applicable skills. These people have helped me a whole lot with the three books I've put out into the world, and I can never thank them enough.
But I haven't sent the stuff I'm working on right now to any of them. And I'm not sure I'm able to say why.
It's not like I've never gotten crit before.
It's not like seeing "Eeeeew!" in the margin deflated my writerly balloon.
It's not like I just want to write my own thing, how I want to write it, and goldarnit people are going to adore my genius, just you wait.
It's not like my CPs aren't craft geniuses. (See above.)
It's more like... see, I don't know. Maybe I was more confident back when I was sending folks pages? I'm not entirely sure what I fear would happen if I shared my current story with them. They wouldn't be mean, and I know their insights could help me create a better story, but ... dunno. I'm still nervous.
So that's my advice, I guess: share when it doesn't scare you. Share when you can handle the eeew in the margin. Share with people you trust because they are seriously skilled writers and/or marketers, and you know they have your best interests at heart.
And this especially: definitely share that manuscript with someone skilled in craft BEFORE you publish. Never, under any circumstances, write a book and then self-publish it before you have someone -- many someones, ideally -- read it. The last thing you want is some reviewer to be your harshest critic when it's too late to fix the book in any appreciable way.
In the mean time, if you're cringing in the corner and hugging your book to your chest, please know I get you. You aren't alone.