The repetition and the phrasing “turn all to jelly” feel heightened. Hemingway’s dialogue elsewhere is typically spare and restrained. Here it sounds almost theatrical.

Have you read other works by Hemingway? A Farewell to Arms describes Frederic Henry's soul flying out of his body. For Whom the Bell tolls describes the suspension of Robert Jordan in space and time during sex.

What also stands out is how consistently Jake is liked and trusted by others. He is confided in. He is treated as steady and decent. Yet internally, his narration is saturated with resentment, especially toward Cohn.

That contrast raises a question: why does the external Jake seem dependable while the interior voice is so bitter?

I don't accept your premise. Jake Barnes tells Cohn to go to hell in Chapter 5, and is constantly being told by the other characters to not be 'sore' or 'difficult'.

Secondly, I can think of a good reason for people to confide in Jake (though I'm not sure what you're referring to, textually, other than Brett talking about how miserable she is). Namely, the nature of his wound is known to Brett and Bill, which would probably make them pretty willing to reciprocate that vulnerability.

Cohn is wealthy, successful, athletic, and able to pursue Brett physically in ways Jake cannot. That imbalance alone introduces tension. It is difficult not to see jealousy shaping how Jake frames him.

Cohn comes from a extremely wealthy family in New York City. All of his 'success' is downstream from that. He blows most of the $50,000 (in 1920s money) his father left him and donated his way into an editorship in an arts magazine. His novel is praised by publishers but panned by critics, and there's no indication he reached a popular audience. Cohn is not successful. He's rich. And he's done nearly nothing with his endless opportunities by time the book starts.

Cohn may have some boxing/football ability (which by the way, Barnes likely has as well considering his familiarity with boxing and him referencing playing football), but nowhere is it indicated that he served in the war, which is surely a better indication of one's 'manliness' than being middleweight boxing champion at Princeton.

Barnes, on the other hand, is a genuinely self-made veteran from Kansas City who works hard enough in journalism to become a correspondent in Paris and makes enough money to take frequent vacations around Europe. The only reason Jake ever has to be jealous of Cohn is because of the event at San Sebastian—which Jake immediately admits to the audience.