Any writer can experience that moment where you suddenly realize that your “idea” of the story isn’t going to get you to the end.
This moment is initially devastating and then, hopefully, liberating as the writer begins to realize that it isn’t her job to know how to get to the end. Her job is to inquire into the nature of what she’s attempting to express, and let her muse come up with the goods.
It’s not just first-time novelists or screenwriters who struggle with plot. Every writer struggles with shedding the idea of the story for the truth of the story.
The solution is to recognize that your plot is simply a vehicle for what you’re attempting to express. Focus on the theme. Stay connected to that, while holding your idea of the novel loosely.