lose (one's) nerve
To no longer have the courage to do something. I did want to ride the roller coaster earlier, but I'm starting to lose my nerve now that we're actually here.
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]