If you’re only past part a, you need to keep looking at the idea until you understand what’s wrong with it, what makes it bad. This probably means you keep working on it until you find yourself in a corner unable to improve the idea/script. Then you’ll be able to do some retrospection and say “Here’s where this went wrong.”

And then the trick is to make sure you don’t make those mistakes again. It’s a lot harder in a creative process than it is other things, but it’s gotta be doable.

And nothing is more valuable than constructive criticism, even if it can be hard to take (I know I’m horrible with it). So if there’s a community out there who can review it and give you useful feedback, leverage it.