I don't know how the company culture differs between Korea and western countries, particularly given that within a country work cultures differ, but if you're being asked to do more than you're able to, you should really say so. Could be your bosses haven't realised you're doing higher priority work for someone else, could be that there's work being assigned twice, could be that they're underestimating how much work a certain task is. Doesn't matter. Their job is to get someone to do the work. If you can't do the work because you're spending that time doing other work, asking you to do the work won't get the work done. And working yourself into a burnout costs the company a worker, meaning they'll have to train someone to replace you. This is inefficient and costs the company money. Now, if you regularly can't do the work being asked of you, then the work might not be right for you. Perfect world, you could go look for a better fitting job and they'd go looking for a better fitting employee, but we live in a job scarcity situation right now so....
But TL;DR: if your boss asks you to do something you can't, say something about it.