If you can't do this for your story then your story is not finished.
Now, I'll be honest with you. In my example that wasn't the exact idea I started with. My process (and everyone has a unique process) is I *think* I know the beginning, middle, and end of my story before I start writing and so I write the first draft. I read what the result says. Usually it says something somewhat different from what I thought it was going to say. I have to decide on the story premise. I revise, including cutting all the stuff that doesn't support the premise. Sometimes I have to add a bit. The point is to create a coherent story.
Your method will differ from this. But... you must ask yourself:
What is your story premise?