The phrase "go off the deep end" means to lose control, become irrational, or act impulsively. There are several antonyms for this expression, including "stay calm," "keep your cool," "remain collected," "be level-headed," and "exercise restraint." These antonyms suggest a sense of composure and rationality in the face of adversity, rather than an emotional outburst. To avoid going off the deep end, it is important to remain focused, consider the situation from all angles, and approach it systematically. Navigating difficult situations with a level head can often yield better outcomes than acting impulsively.