We have used Celtx a few times now. It's a great small tool, mainly because you can put together the script, story board, character definitions, actor details... all that and taking less space than the smallest MSWord file.
It also speeds up the writing as you don't have to bother about the format or character names so much.
On writing the first script, even if it's short, it's true what everyone says about scripts: rewrite, rewrite, and when you think you are done then rewrite.
A bad script always ends on a crap movie.
One thing that helps a lot is the knowledge of editing techniques and camera techniques at the time of writing a script. As you said, you write more theatrically, and that's probably because of your background. I had the very same experience. Then the first time we were filming a short we discovered that we didn't have to limit ourselves so much!
Also allow other people to read it, don't take in all the comments but listen to everyone. If people don't understand something (like why is the character acting like that) then people will be confused when watching the film too, and if I am confused I stop watching.
It's great that you finish what you started, now push and try to make it happen, but remember it won't look the same as you pictured it, but that doesn't mean it's wrong, on the contrary, it means that it has a life on its own.
