At what point do we stop worrying?
I suspect never. Afterall each rewrite brings something new, more polished, to the table.
Okay, so you've found your producer, they like it, they want to make it. They give you notes. You tweek accordingly. They appear happy. It's good. They vanish off to locate funding.
Which gives you time to think....
And on reflection you know the story can be better.
If you rip half the story out and start all over again, potentially the next version will knock your - and everyone elses - socks off. ... Or it might not.
The producer is happy with what is there. And there's no guarantee that the new version actually will be better. You might wind up destroying the certain something that attracted them to that particular story in the first place.
Bit of a tight rope to walk.
So you don't. You leave it. And you worry.
You really don't need to.
Either the film will be made, or lets face it - smoke and mirrors - won't be made. It's out of your hands.
All writers are control freaks. Let's face it, crafting a story - influencing your characters lives ... feels pretty close to being a divinity. Even if only in the world you've created.
Being powerless over the fate of your story => fear => worry.
We all go through that fire, that torment. Part of being a writer.
I think once the producer has said yes, they like it, they want to make it, ... I think you are allowed to stop worrying and move onto the next project. Relax, have an hour or two off, and then dive into the next story.
Fires of creation burn better and hotter than the cool fire of torment.
Have at it, says I. On to the next inferno! :)