I was so angry with him that I almost lost it.

‘Almost lost it’ means the person almost lost control of themselves by saying something or doing something they really did not want to, like calling the person a bad name, deliberately knocking things over and making a mess, throwing something at the other person, or, hitting the other person.

他讓我十分生氣，我簡直忍不住要發飆！

Almost lost it指瀕臨「發飆」「抓狂」的邊緣，快要控制不了情緒了。假若怒火延燒，無法及時冷卻下來，有可能就要摔東西、罵髒話，或動手打人了。