The phrase "struggle along under" communicates a sense of difficulty and burden. Some synonyms that capture this same sense include "plod," "trudge," "tread," "labor," and "drag." Other synonyms may emphasize the uphill nature of the task, such as "grind," "lumber," "heave," "shoulder," or "slog." Additionally, some synonyms may convey a sense of holding on for dear life, such as "cling," "clutch," "grasp," or "clamber." Regardless of the specific synonym used, they all communicate that the task at hand is heavy, challenging, and often overwhelming.