Writing really is one of the strangest undertakings. We are seduced by a story idea in our head and practically driven crazy until we succumb and begin the process of putting it on the page.
And then, the real torture begins. We manage to write a scene or two, or even a whole first act; and then, inevitably we begin to second guess ourselves. As we soldier through the next tough scene sequence a little voice begins to speak over the one in our head telling us the story. 'The main characters name isn't strong enough - you should FIX that. The villain isn't evil enough; the opening scene isn't FRESH enough. The scene transitions'.... well, you get the idea!
Some of this self-commentary may actually be quite accurate. Your main character does have a putzy name, at the moment. And maybe those scene transitions do really suck. But guess what, we don't care - NOT RIGHT NOW. Your only job in the FIRST DRAFT IS TO FINISH! Repeat this to yourself, over and over and over again when your inner critic dares to speak. Resist the temptation to go back and just KEEP MOVING FORWARD; that's the only way to finish your first draft.
On the other hand, don't lock up that inner critic and throw away the key. Just throw him/or her a bone to keep her quiet. One of the best gimmicks that I've discovered is keeping a notebook and pen next to your computer and every time that critic SPEAKS - write it down!` Voila, your inner critic feels 'heard' and so deigns to leave you alone so you can write. AND, bonus time, by the time you have a first draft done you also have a dandy blueprint for your first REwrite waiting for you as soon as you type THE END.
I hope this helps. And I wish you lots of luck on your screenplay. Let me know when you've finished. I'd love to see your work!