the end of the (line|road)

the point at which making further progress becomes nearly impossible

The idiom “the end of the line” is believed to have originated in the 19th century, when trains were the primary mode of transportation. When a train reached the end of the line, it could not go any further. Over time, the phrase came to be used in a figurative sense to describe any situation that is at its conclusion.

After numerous attempts to revive the failing project, the team finally reached the end of the road and had to abandon it.
After years of trying to save their marriage, they finally reached the end of the line and decided to get a divorce.
The athlete's injury was so severe that it marked the end of the line for their professional career.
The company's financial troubles had been mounting for years, but when they lost their biggest client, they knew it was the end of the road.