To compose oneself so that one's actions are not dictated by one's emotions. I know you're angry, so you need to collect yourself before you go in there and say something you'll regret to your boss.
See also: collect
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
collect yourself/your thoughts
1 try to control your emotions and become calm: I’m fine — I just need a minute to collect myself.
2 prepare yourself mentally for something: She paused to collect her thoughts before entering the interview room.
Farlex Partner Idioms Dictionary © Farlex 2017