Wiktionary
collect one's thoughts
alt. (context idiomatic English) To become mentally composed, especially after being distressed, surprised, or disoriented; to become calm or organized in one's emotional state or thinking, as in preparation for a conversation, speech, decision, etc. vb. (context idiomatic English) To become mentally composed, especially after being distressed, surprised, or disoriented; to become calm or organized in one's emotional state or thinking, as in preparation for a conversation, speech, decision, etc.
Usage examples of "collect one's thoughts".
He had not been listening very attentively, the glare of the sun in the shadeless valley was altogether too strong, it was difficult to collect one's thoughts.