Earlier quoted context omitted.
Say you're planning to write a letter (or an email), but you're not actually sitting at your computer to do it right at that time. You're in the shower, or walking around or whatever. Will you play out specific sentences you intend to include, in your mind? Could you do that if you wanted to?
Not OP, but their description fits mine perfectly. When I write, I don't know what will come out until it does. This goes for this comment (funnily, typing this is not harder because I'm aware of the process - it just pours out) as well as the novel I'm writing. When I'm not writing the novel, but thinking about it, there are two different stages: "thinking about the chapter I plan to write" when I form the sentences…
I mean, I doubt I could keep more than a few sentences in my head without any mistakes, although if I were to mentally rehearse more than that they would likely come back to me to some degree as I went to write it out. As you form the sentences, is it like silently talking to yourself (how I'd describe my experience)? Or something else?