The phrase "lets oneself go" generally means to stop caring about one's appearance or behavior. Some synonyms for this expression include "loses control," "falls apart," "gives up," "abandons oneself," "surrenders," "degrades," "falls into disarray," "deflates," "unwinds," and "gives in to temptation." These phrases all imply a sense of giving up control or buckling under pressure. It's important to note that not all of these phrases have inherently negative connotations, as sometimes giving oneself permission to "let go" can be a cathartic release and lead to personal growth. Ultimately, the synonym one chooses will depend on the context and tone of the sentence.