There’s value in meeting adversity head-on, and there’s a lot to be learned from the process.
One thing you must learn is that your ego, or self-identity, is not the same as what you’re trying to achieve.
If you're writing and publishing a book or designing and selling another product online, you'll get feedback from those who purchase your book, backpack, or whatever you offer for sale.
Some of that feedback will be positive, some will be negative, and some will be downright mean and insulting.
You must learn that you are not your product. If your product/book/whatever, gets panned, that does not mean that you have been rejected. Don't fall into the trap of being depressed by negative comments.
If the feedback is insulting—or worse, a personal attack—then the person leaving it has problems that have nothing to do with you.
Remind yourself of that instead of getting upset and/or angry.
Like so many things in life, this can be easier to say than do, but work hard to separate your ego from what you create and offer to the public.