the limiting factor was the bit of my brain through which knowledge has to be squeezed to turn it into linear sentences

I get this after about an hour no matter what I'm trying to write. It's one reason I dumped all my essay subjects. It irritates me a lot because I have a whole load of things I'd like to write, but no ability to do so: I have the knowledge, I even know approximately what I want to say. But turning it from fuzzy concepts to logical sentences just doesn't work.