"For a moment a phrase tried to take shape in my mouth and my lips parted like a dumb man’s..."

"... as though there was more struggling upon them
than a wisp of startled air."

That's today's sentence in the "Gatsby" project, in which we isolate one sentence, each day, from "The Great Gatsby" and try to figure out what the hell is going on.
This one isn't hard. It's just a guy trying to talk. Imagine how you would write this sentence if you only wanted to express: I had trouble talking. There's only a slight chance you'd end up with a sentence this long. And if you did, could you have sustained the drama like this, from moment to air? What lightness in moment and a phrase trying to take shape, then blah blah blah wisp of started air.

The narrator is struggling to speak, and the author is struggling to drag out a long sentence about this struggle. It's a sentence that begins and ends in lightness and that has heaviness in the middle. I'm counting 3 things as heavy: 1.  the near redundancy of mouth and lips (each with its own my), 2. the intrusion of the unwanted character a dumb man, and 2. more struggling upon them than.

There was nothing but air on the man's mouth-and-lips, but it seemed like more could be there, struggling.

ADDED: Chip Ahoy animates the sentence: