Okay, so you've done the hard work and outlined your screenplay. You devoted the necessary time to create complex characters, and you labored to develop a detailed story; however, you didn't know when to stop. You tweaked it, and tweaked it, eventually even trashed it, outlining again, and again, and again. But why? Because you wanted to make your outline perfect. Big mistake.
An outline is a comprehensive guide – an essential part of the process - and many would argue the most important part, but by no means is it some magic recipe that is structurally perfect. There comes a time when outlining is for the birds, and you just have to buckle down and do it: write the damn script.
But when you start writing, remember to knock out the first draft as fast as possible. Your first draft is never really good - to be honest, it shouldn't be; that's why we rewrite - but you have to get it down on paper. So if you've been outlining for too long, don't waste any more time now. Just write. Let your characters breathe and allow the story to gain some legs.
And after you complete the first draft, really take a good look at the product - look at the broad strokes of story and character. It is very possible that you must conclude the next step is a page-1 rewrite, but don't be upset. You had to write the first draft to figure that out - to discover what the story is REALLY about.
You might realize that during the writing process that the true story doesn't start until the third act, or that a supporting character is actually your main protagonist, or that you have the wrong story in the right world – all that is fine, as long as you see it and make the necessary changes, even if that means writing a brand new outline.