I'm probably not the best person to speak on this since I've spent about three years working on my comic and haven't even gotten to scripting yet. xD I think experience probably helps, but the writing process is different for everyone. Some people spend a few months or years on scripts, other people finish them in like a week. It just depends on what kind of writer you are.
As a perfectionist, I understand how much longer things take when you're always changing them. It's hard for me to stop changing things because every time I feel satisfied with something, a better idea comes along and my previous satisfaction isn't enough anymore. If that's the same problem with you, I'd recommend a few things.
Do not rewrite as you are writing your script. When you say 'rewrote', I don't know if you mean that you rewrite while working on the script, or if you finish an entire script and rewrite it afterwards. If you're rewriting as you write the script, you'll probably never get it done. Not only does writing an entire script give you something concrete to work with, it forces you to commit to your original ideas and see if they really need to be changed or not. It's easy to see all the flaws in something as you're writing it, but once it's done, you might find that it comes together nicely, or that it really doesn't need that many changes.
Keep every finished script you make and compare them. Print them out, comb through each one, and mark what you like and what you don't like. Focus on changing the things that consistently don't feel right to you. Like, if you've been through each script and you keep noting that the main character seems too bland, that should be something you focus on changing instead of other minor things. Do not just get rid of your old scripts and replace them with new ones; I don't know if that's what you've been doing or not, but you should really keep everything you write. Hell, if you're on Microsoft Word or if you're writing on paper, you can either slash out sections you don't want instead of erasing them. That way, you can come back to those old ideas if you change your mind or want to work off them.
And finally, get someone to read it if you can! I know how stressful this can be, but it really does help to have an opinion aside from your own. Obviously your satisfaction matters, but getting the reader's opinion is just as important and will often help you pin down what needs to be changed and what should stay the same.
These are just suggestions based on my personal working method and what I've read online, so obviously make sure you experiment and do what's best for you. Good luck!
