The first draft is not the finished book
A common misconception is that you sit down. You write. Out comes magic. Once you've finished the first draft you're done. The book is finished and there is no more to do about it. If it's bad, you're a bad writer.
Nothing could be more wrong.
Yes, we all wish we could write first drafts that were gorgeous... (or at least many of us do).
But a first draft is more like a test you take, and then the second draft you get to retake that test, and then a third, and a fourth time. Up to a handful or so of drafts you're more or less guaranteed to make it better.
At least if you know how to edit your drafts and rewrite them. For that, I recommend you start with James Scott Bell's "Revision and Self Editing for Publication". There are also tons of resources on the net.
The key takeaway is that you need to edit your first draft to make it better. It will not be good from the get-go.
After all, if our first drafts were, as a rule, published, some bastard would come along, write a second draft and blow us all out of competition.
But how do you deal with your inner critic?
Here's a "secret" to dealing with any form of disturbing thoughts.
You don't. You can't even.
Ever found yourself trying not to think something just to find yourself thinking of nothing but? Let me give you an example:
I want you to not think of a red fire truck. One with blue blinking lights and a silver ladder. As it screams by you on a street in your hometown on a sunny spring afternoon. Do not think of that...
You thought of a red fire truck.
The trick is to know that thinking is not doing, or for that matter manifesting. (If indeed a red fire truck did roar through your living room as you read this, you're a Marvel character and you need to stop snooping on us writers... ;o)
And just like you're not always in control of your thoughts, your inner critic will pop up and start telling you how everything sucks.
My theory is that your inner critic comes from your self-preservation. It's trying to protect you from harm. And sure, writing can be dangerous, but hopefully, you live in a part of the world where it's about your self-image, your pride, not your physical body. That none of the harm that could come to you would break any of your bones...
And just like you cannot not think a thought, you for sure cannot turn off your self-preservation. Thankfully!
The trick, instead, is not to push through, push away or push down the inner critic, instead, you need to listen to your inner critic/your self-preservation.
Imagine your mind is a stage, you, your consciousness, your "inner eye", is sitting in the audience, and entering the stage are all these thoughts and feelings.
You invite them onto that stage so they can tell you their thing.
Listen to them.
Describe what they have to say using objective language. Do not value it as good or bad, helpful or unhelpful, smart or stupid. Just pick up what the objective message is.
You might want to write it down. For instance as a note/comment in the manuscript.
Perhaps it's that you'll get bad critics. Perhaps it's that everyone will hate what you're writing, that it will be bad, that no one will care, etc.
Just take it in. Then let it be for a while. Take a break if the feelings are strong, keep writing, or go for a walk. Try not to come up with criticism, a plan, or anything else, just feel them and see what that feels like.
One of several things might happen:
The thought might go away and leave you without even feeling worried.
You may realize you are taking some kind of risk and figure out ways you could prepare yourself to deal with it better.
Or you find this is just a brain function specialized on predators sneaking in the bushes and fires, and it is taking control for a few moments.
Listen to the warnings, it is after all your self-preservation speaking, but know this; your self-preservation was made to protect you from physical danger.
It makes sense when dealing with physical danger to avoid it, to not go toward the danger, but instead run away from it...
Unfortunately, doing so with regards to non-physical danger (like failures at work or in your social life or anything else that will never escalate into physical danger—e.g. flocks of pigeons on town squares...) is at best going to limit your life experience, at worst cause phobias and all kinds of other problems.
Your self-preservation does not know this, and you cannot turn it off, so your best bet is to listen to it, let it come on the stage and tell you about the dangers, and then get the objective view of them, what is actually going to happen? Will my hair catch fire? Will I become homeless?
You can always ask, is this going to cause physical harm to me? Is my physical life in danger? Will it make me bleed? Will it break my bones? Unless you're writing some really heavy stuff the answer should be no, although a lot of brave people out there in bad places wouldn't even let those dangers stop them...
The thing is, once you've listened to your self-preservation enough, it seems it's happy that you have and will quiet down.
As long as you really listen to it and don't just listen to try to make it stop. It will of course know if you do. It's after all just another part of your mind. And your self-preservation has been trained for billions of years not to take that kind of negligent BS.
You may even be able to listen to your inner critic and keep working on the text (or anything else in your life it wants to warn you about) at the same time.
To read more about this, "technique" and similar thoughts, check out "Get Out of Your Mind and Into Your Life" by Steven C. Hayes.