In "Across the Bridge" by Graham Greene, what is a proverb that relates to the disappointment at what is found on the other side of the bridge?

Let me suggest "The grass is greener on the other side."

Throughout the story, one of the bridge's functions is as a symbol of a division: a border, something that people expect to bring on major changes and improvements once it's crossed. Let's take a quick look at the evidence in the text for that idea:

After his bottle of beer, he would walk down between the money-changers' huts to the Rio Grande and look across the bridge into the United States: people came and went constantly in cars. Then back to the square till lunch-time. He was staying in the best hotel, but you don't get good hotels in this border town: nobody stays in them more than a night. The good hotels were on the other side of the bridge: you could see their electric signs twenty stories high from the little square at night, like lighthouses marking the United States.

And:

He was disgusted; he had had some idea that when he crossed the bridge life was going to be different, so much more colour and sun, and--I suspect--love, and all he found were wide mud streets where the nocturnal rain lay in pools, and mangy dogs, smells and cockroaches in his bedroom, and the nearest to love, the open door of the Academia Comercial, where pretty mestizo girls sat all the morning learning to typewrite.

So what we see here is the notion that although we expect great things on the other side of the bridge, those great things don't actually happen; the result is disappointment.

One proverb that expresses this idea is that "the grass is always greener on the other side." It means that it's a common error in human thinking for us to look at a different place or a different set of circumstances and assume that it's much better over there, but much of the time, we realize we're wrong. 

If you look around online, you'll see that other languages and other regions express the same proverb in many other ways. Here are some examples:

• The harvest is always richer in another man’s field.


• The apples on the other side of the wall are the sweetest.


• Our neighbor’s hen seems a goose.


• Your pot broken seems better than my whole one.

Other proverbs specifically mention bridges, but these don't seem to connect in a meaningful way to this story. For example, there's the idea that you shouldn't "cross a bridge until you come to it," meaning you should wait until problems actually happen before dealing with them or worrying about it. And there's the saying that we shouldn't "burn bridges," or destroy relationships or options that we think we will never need again (because we might).