lose it

lose the plot {esp. UK} AND lose it (stop being rational about something; become very angry, lose one's cool; become confused, not know where one is going or what one is supposed to be doing) — о любом виде потери контроля или самообладания: выйти из себя; потерять самообладание; сорваться; сходить с ума; растеряться

losing it (losing emotional control, losing the ability to do something, оr losing mental powers)

Example 1: It was too much. She lost it. — Это было уже слишком. Она не сдержалась. / Она не сумела сдержаться. / Она сорвалась.

Example 2: She sat there calmly, biting her lip to keep from losing it. / She bit her lip to keep from losing it. — .... кусая губы, чтобы не взорваться/ не сорваться/ не потерять самообладания.

Example 3: When he criticized you, I started to lose it. I almost hit him. (thefreedictionary.com)

Example 4: He was afraid he was going to lose it, so he cut the meeting short.

Example 5: I just can't deal with this many visitors — I must be losing it. (thefreedictionary.com)

Example 6: After 35 years of teaching, he was losing it. He was confused. (thefreedictionary.com)

Example 7: I thought I was losing it when I couldn't remember the words to that old song.

Also
lose one's reason
lose one's grip
lose one's temper

(fail to keep one's composure; fail to maintain control or one's ability to function || lose the ability to think clearly, become forgetful)

Example 1: When Billy broke the window, Dad just lost his grip and let him have it. (thefreedictionary.com)

Example 2: Ted wasn't running things the way he used to, and his boss thought he might be losing his grip. (thefreedictionary.com)

Example 3: After only a few weeks as a teacher of very badly behaved children she realized she was losing her grip. (= she was losing her control) (english-test.net)

Related vocabulary:
[lose one's mind]

Lose the plot seems to come originally from the idea of an actor, or even a writer or director who gets lost and forgets how the story is supposed to develop. This one of many phrases which are typically used to criticise those who should know better - experts, people in power, people who are dominant in relationships. Also losing it ('it' could also be temper / control), 'he just lost it' means someone has become uncontrollably angry or agitated. (Buzzword)