The phrase "let go by" refers to releasing or dismissing someone or something. The antonyms or opposite words for the phrase might include words such as "keep," "retain," "hold onto," "preserve," or "maintain." These words suggest that one wishes to retain or keep something, rather than release or let it go by. For instance, if one were to "let go by" a potential opportunity, one might instead "retain or seize" that opportunity. If one were to "let go by" a negative emotion, one might instead "preserve or hold onto" a positive attitude. Overall, the antonyms for "let go by" emphasize keeping or holding on, rather than releasing or dismissing.