As the saying went, among major powers and large regional organizations of comparable size, the middle class in the United States had it the worst, bar none. That was why their middle-class anxiety was the most severe.
As for climbing upward, as long as there was still a relatively clear path for advancement, anxiety could be eased quite well. The middle classes in some developing countries were not as anxious because their channels for upward mobility still existed—because their societies were still in an upward stage of development. If they caught the right wave of the times, then one generation's hard work could completely transform an entire family.
By contrast, in some old developed countries, those upward channels had already been completely blocked. Society had been subdivided horizontally into too many tiny slices; whatever class you were in, you did the things of that class. Even if some new opportunity appeared, it would never fall to the people below, so it did nothing to ease anxiety.
As for falling downward, the old developed countries handled that part relatively well precisely because class solidification meant each stratum only mixed with its own people, and assets generally fluctuated within a limited range. It was hard to become fabulously rich, but it was also relatively hard to plummet several levels in one go. At most, you became the bottom of your own finely divided tier, and life was still livable.
But in some developing countries, that was much more terrifying, because if you fell too far and ended up somewhere with poor public order, you might actually lose your life. That would create serious anxiety.
From this, it was easy to see why middle-class anxiety in the United States was so severe: they had both disadvantages at once—"climbing up is very hard" and