lose one's grip
Also lose it.
Fail to maintain control or one's ability to function, as in Ted wasn't running things the way he used to, and his boss thought he might be losing his grip, or I thought I was losing it when I couldn't remember the words to that old song. The first term dates from the mid-1800s, the slangy variant from the mid-1900s.
Fail to keep one's composure, as in When Billy broke the window, Dad just lost his grip and let him have it, or I just can't deal with this many visitors—I must be losing it. [Slang; first half of 1900s] Also see lose one's temper.