People often imagine it's easier for me to write than talk. No, one advantage to writing is that people don't have to sit around and wait while I do it -- but it's a more cumbersome.
It's not just a matter of being strategic or finding the right thing to say, though that is definitely a part of it in certain contexts. But there's also the issue of putting it all together. And sometimes I would describe it as a cognitive thing more than a psychological one. Words that go around and around my head before I type them -- that's obsessive compulsive, huh? Not always. Anxiety, perfectionism... those are not the only precipitating factors. One factor that seems to precipitate it is being tired. And one thing that can help is a stimulant -- e.g. caffeine.
The more tired I am the more I seem to resist task switching: like switching back and forth between purely cognitive things (composing) to things that involved orchestrated motion. I want to compose a whole paragraph in my head while lying down -- and rehearse it, too. I might feel ready to type when I've got the paragraph to the point where... well, where it pours out almost as fluently as 3 x 3 = 9.
You know those old commercials where they go, "Hey, your chocolate is in my peanut butter!" It's like the part of my brain that orchestrates expression and the part that controls motion don't want any mixing. There's a time for chocolate. There's a time for peanut butter. But not both.
And I may compose the "Hey, your chocolate is in my peanut butter!" quite fluently while having a conversation in my mind with a friend. (Which is how I compose a lot of things.) But it can still feel less daunting to keep playing it in my head than to actually switch gears and it.