keep temper

keep (one's) temper

To refrain from becoming angered, enraged, or upset due to some provocation; to maintain control of one's composure despite being angry or upset. You're a good driver, but you need to do a better job of keeping your temper when other drivers make mistakes. It's hard to keep my temper with the kids after being kept awake by them all night long.
See also: keep, temper
Farlex Dictionary of Idioms.

keep one's temper

 and hold one's temper
to hold back an expression of anger. (The opposite of lose one's temper.) She should have learned to keep her temper when she was a child. Sally got thrown off the team because she couldn't hold her temper.
See also: keep, temper
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • hold (one's) temper
  • hold one's temper
  • keep (one's) temper
  • keep/lose your temper
  • get (one's) bowels in an uproar
  • get bowels in an uproar
  • get one’s bowels in an uproar
  • a short fuse
  • I don't want to upset you, but
  • tell a (little) white lie
References in periodicals archive
The final bookings make-up was 90 as Korean referee Kim Young Joo struggled to keep tempers in check as the Turks reacted angrily to Rivaldo's play-acting.
And there are strict ground rules to keep tempers at bay.
A well thought-out plan will avoid frustration, keep tempers cool, and save money by handling materials only once.
The referee and match officials had to intervene to keep tempers at bay.
INDIAN ARMY responded by keep tempers under check but promised a response at the time and place of its choosing.
I suggest you consider taking similar action if you want to keep tempers cool.