When you lose your temper, lose your cool or lose your head it means you have become uncontrollably angry or to fail to control your emotions or keep your composure and become angry instead. To keep your cool or keep your head means the opposite to remain calm and composed in difficult, stressful or upsetting circumstances. 

1. When I feel that I'm on the verge of losing my temper, I take deep breaths to try and calm myself down.
2. Sometimes it's hard to keep your cool when everything is going wrong around you.

1. Helen is quick to lose her temper whenever someone criticizes her. She needs to stop letting people get under her skin.
2. My boss lost his head when I told him the project wasn't finished.