The antonyms for "lighten upon" depend on the context of the phrase. If "lighten upon" refers to the act of discovering or stumbling upon something, the antonyms would be "miss," "overlook," "ignore," or "forget." Alternatively, if "lighten upon" refers to the act of adding weight or burden, the antonyms would be "remove," "lessen," "reduce," or "unload." In either case, the opposite of "lighten upon" would imply a failure to find or a tendency to add excess weight, responsibility, or stress. By using antonyms, we can highlight the negative consequences of these actions and suggest alternative solutions to improve our experiences.