lose one's nerve
Become frightened or timid, lose courage. For example, I wanted to ski down the expert slope but then I lost my nerve. This expression employs nerve in the sense of “courage or boldness.” [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.