낙심하다. To be discouraged. (Yes. 낙심하다 can mean either to be frustrated or to be discouraged. But if you presented more context I could help you clearly. Because when I learn English, I learn the usage and the expression of words. I don't strictly match Korean words to English words as one Korean word can indicate more than just two or three meanings, and sometimes it has more than 8 meanings like English.) 낙심하지 마세요. Is usually used when you encourage someone who failed to do something. So you can say to a person if he or she failed. 낙심하지 마세요. or you can also say. 힘내세요. 빠지? 매우 빠지는 했어요.(? It doesn't make sense.) it was very discouraging. (그건 정말 실망스러웠어요.) 실망하다 and 낙심하다 have similar meaning each other, but there are some invisible rules can be used by context. Hope it helped you somehow.

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