This is so fundamental it's ridiculous, but a screenwriter can never hear it enough times.
You finish a first draft to see how good or bad your screenplay is. If it's more good than bad, you commit to completing a second or third or fourth draft until it's finally done. If it's bad you have 3 choices:
(1) Decide if it's worth rethinking
(2) Put it aside for the time being and let the idea germinate
(3) Junk it permanently
Don't be ashamed to junk it permanently. It's better to cut your losses before you get even further into the miasma of trying to make something unworkable work. Kind of like the economists say: don't put good money after bad.