How do I deal with overthinking when I write?

You need to stand back from your creation before you do any writer’s amendments to the manuscript.

I don’t think it’s overthinking when you pare down sentences so every word counts, and substitute words in phrases or sentences to make things flow better. This is the essence of writing: rewriting and rewriting again. Yes, it suits slightly obsessive types, but if it gets you the best possible version of your script, then that’s what it takes.

The more reading and writing you do, the less rewriting you will need to do.

Make certain you get a professional final edit though.