This is the first time I thought about a difference between 드디어 & 결국. Very good question, 알렉산드라! I'd like to share what I figured out so far after doing some search here and there. Hope it helps you! 드디어(순우리말) vs 결국(한자어) 드디어 You can use 드디어 to suggest that something happens after a long period of time, usually later than you expected it to happen or to indicate results of some actions or events. 예) 드디어 시험이 끝났다. The exam has finished finally and we can assume that maybe the person who spoke has been longing for the end of the timeline by passing through a certain amount of time. 결국結局(맺을 결, 판(상황) 국) You can use 결국 to indicate that what you are saying is a somthing that had to happen in the end. 예) 그는 결국 성공했다. He eventually succeeded and this sentence tells us that his success was supposed to come true at the end.

드디어 : In the state of something that one has eagerly waited for being achieved eventually 결국 : As the result of something. 드디어 is used for positive situation 결국 is used for any situation Therefore, if the result is good, use 드디어 and if the result is bad, use 결국 e.g. 드디어 지긋지긋하던 시험이 끝났다(O) 자주 싸우더니 드디어 이혼했군(X)ㅡ>결국

결국 = in the end; as a conclusion. (= 마지막에는). 드디어 = finally; at last. (something expected is finally happening - doesn't have the sense of conclusion 결국 does) There is also 마침내 which sits between 결국 and 드디어, probably closer to 결국. e.g. - 오랜 도피 끝에 범인은 결국/마침내 체포되었다 (The suspect has been arrested at last after a long hiding). - 드디어 축구 결승전의 날이 밝았다 = The day of the soccer final game has dawned at last.