"Held it down" is a phrase commonly used to describe someone who has stayed strong, maintained control, or persevered through a difficult situation. Its antonyms would be words that convey the opposite meaning, such as "lost control," "crumbled," or "surrendered." These terms suggest a lack of resilience, and an inability to withstand pressure or adversity. Using these antonyms in a sentence could convey a sense of defeat, weakness, or vulnerability, as opposed to the strength and composure that "held it down" implies. It's important to consider the context when using these antonyms, as they can dramatically change the tone of a sentence.