The phrase "gone deep end" typically refers to someone who has lost control of their emotions or actions. Antonyms for this phrase could include "calm and collected," "rational," "level-headed," or "composed." These words all suggest a sense of balance, control, and stability, which are the opposite of the frenzied, unpredictable behavior associated with going "deep end." Other antonyms might include "reserved," "quiet," "introspective," or "cautious," which all suggest a more measured approach to handling emotions and situations. Ultimately, the antonyms for "gone deep end" all share a common emphasis on maintaining a sense of self-control and balance, even in the face of challenging circumstances.