When the going gets tough they buckle like a belt

When facing adversity, some individuals rise up to meet the challenge. This antonym of a more well known phrase describes much the opposite—a person who cannot handle pressure and crumples (buckles) in the face of hardship, frequently leading to their own failure.

"I would say 'when the going get tough, the tough get going', but even though she studied for weeks, when Anna went in for the final test, she totally blew it."

"I'm not surprised, it's Anna after all. When the going gets tough, she buckles like a belt."
("When the going gets tough they buckle like a belt used" when in a sentence may include pronounces, or a name instead.)