"Kill your darlings" only applies if you're holding onto something that actually hurts your story. But jerks sometimes use "kill your darlings" as a weapon to try to break you. Don't wantonly kill what you love the most about your story. That's why you're writing it. Make it work.
So, so true. Not often, but it does happen. E.B. White used to struggle with this.
The only way you can figure out which it is is through many years of practice.
100%. I've found the longer I've done it, the more I've realized that it's not about not failing -- it's about failing faster. And then getting on with it.