If you’re editing your own work, how do you stop yourself from over-editing? To me, it seems like there becomes a point where the editing is noticible in a negative way on the screenplay.
Editing is a big part of any writing, but it’s uniquely important in screenwriting, where so few words have to stand for so much. The most important thing to keep asking as you edit every word, line and scene is, "Do I need this?" or more to put it more fully, "How important is this to the story I’m telling?" Often whole scenes and sequences are felled this way, and it’s better (and cheaper) to do this kind of work before you’ve spent millions of dollars to film it.
Where editing can go too far is in sacrificing tone or flavor (especially humor) in service of story efficiency. Good writing has bumps and curves, and not everything should flow in the fastest possible way. Screenwriting is like any art that way: deciding what is too much and what is too little, in order to know what is just right.