I've written a large section of my book. The questions that keep haunting me are: Is this story unique? Will people like it? I think it's becoming repetitive. Will the characters be loved? And a lot of questions.
Author critique groups are valuable. I have hosted one for years. Hearing what other writers have to say about your writing is useful because writers, in my experience, tend to be able to give a little more explanation of the reasons for their opinions than readers at large can. And certainly don't limit yourself to feedback from other writers. Consider anyone you think can be thoughtful.
Feedback from others is helpful, but don't be a slave to it. When someone objects to something you wrote, do your best to figure out the "real" objection. If someone disagrees about a character you killed, for example, it may be a sign that you didn't communicate what you meant to through the death but that the death itself was still the right choice. And always remember that criticisms of your writing are not criticisms of you personally.
By showing it to someone and getting feedback on it. Then you will know answers to at least some of your questions. You can't answer them yourself, someone else has to answer them for you.
Regardless of what the outcome is, it will be liberating, because then you can move on to acting on their feedback. Continuing forward, or fixing things that aren't working.
Happens A LOT. To many writers. A writer's opinion of a work when it's being written is very unreliable.
Just write on. In the immortal words of James Thurber, "Don't get it right, get it written."
At the very least, you need practice writing. If it turns out that your book is as bad as you think it is now, nevertheless you will have learned something in the process that will enable you to write better books in the future.
After you finish it and put it aside for a month or two, you may find it's better than you thought. You may also need to show it to beta readers to get a clearer view on it.
This video of Ira Glass (from This American Life) gets me through that problem every time.
By the time you first begin writing, you've already developed good taste, being able to tell good work from bad. So in your early work you can see clearly how far it falls from the best stuff you've seen. You have to recognize that gap, but also recognize that everybody has one of their own. And know that you'll improve and narrow that gap with practice.
As with anything in life, you just do. If you're not good at it, you'll get where you want to be, with time. Write, write, write. It doesn't matter if what you write is terrifically bad, short, if it has lots of errors in it; because that's the only way you'll learn.
The fastest way to learn is to fail.
I started out writing very, very short stories -300 words were a lot for me- that then turned into 10-pagers, and some day stories that I actually enjoyed. The important thing here is the mindset. It's not going to be perfect, nor are you going to write a phenomenal book in your first year of writing. Be realistic, but also do what you like to do, don't force yourself to be perfect with the first try.
You have to enjoy the process, not necessarily the result.
Write ahead, don't spend your time on going back and revising, rearranging and correcting your text, nobody needs it to be perfect if you don't expect it to be. Everybody writes a lot of nonsense, poorly written characters, stories that lack depth. But that's how they get to write amazingly interesting characters, witty jokes and mind-blowing plot twists later on.
I'm no man to say my writing is perfect, but again, I don't care. I enjoy writing a lot and that's what matters to me. I love it and am never going to stop. Sometimes I show something to my friends that I'm particularly proud of - but I don't get too discouraged if they say they don't like it.