Just don’t ask. Do you really care if your friend, or an expert, or anybody that reads a few pages of your book thinks you should quit? Does really helps you if they say you should keep writing?
When you’re alone with your keyboard, paper page, or recorder, does it really matter if the others support you or not?
No! It doesn’t! You’re always alone when you write.
Instead, ask a different question: How can I make this book better?
Move beyond your doubts. You’re a writer. Act accordingly!