One way that the old man (Santiago) struggles with his profession is that fishing can be a feast or famine line of work: some days or seasons can be great and others can be catastrophically terrible. Having good days and bad days is common for any person, but a fisherman like Santiago has far less control over whether or not his day on the ocean will be fruitful or not. He can go out one day and catch a bunch of fish, but on the next day, he could do everything exactly the same and catch nothing. He can ceaselessly work hard at his profession, but if the fish aren't biting, he's not catching.
Santiago has been a fisherman his entire life, so having days without a catch are not unknown to him. What has got to be unbelievably frustrating is that he hasn't caught a fish in 84 days. Santiago has deep reserves of perseverance and hope to keep going out day after day despite repeatedly coming back with nothing. I think readers can find that aspect of Santiago meaningful. Jobs can be a tough slog, and sometimes a person doesn't immediately see the payoff and benefit of going through hours of unsung work. Santiago teaches readers what Joe Dirt from a 2001 movie also taught: it's important to "keep on keeping on." In other words, never give up. Santiago may have come back with nothing again, but that won't stop him from going out the next day.