This one indeed features an extremely advanced grammatical structure. 1. [(((어린 나이에) (끝까지) (본인의 인생을) (충실히 살다 가려고)) -> (애쓰는)) -> (그 모습에서)] 2. [((가방만 들고) -> (왔다 갔다 하며)) -> (시간을 허비하고 있는)] -> 3. 내가 얼마나 한심한지 깨닫고야 말았다 The whole sentence is actually not that hard if you omit 2) 1) is the sub sentence: "Seeing (the little girl/boy?) doing her/his best to live her/his life diligently until the bitter end at a very young age" 3) is the main sentence: "I ended up realizing what a pathetic person I am, " 2) is the sub sentence describing that 'I' "just wasting time running around mindlessly with a bag in the hand" (implying commuting to school/college at the parents' expense) Seeing (the little girl/boy?) doing her/his best to live her/his life diligently until the bitter end at a very young age I ended up realizing what a pathetic person I am, just wasting time running around mindlessly with a bag in the hand.

My translation is =================================================================== Seeing him(or her) struggling for a fulfilling life even in his(her) young age, I realized that I was a pathetic creature with a commuter bag just wasting my time. ===================================================================

어린 나이에 끝까지 본인의 인생을 충실히 살다 가려고 : I wanted to live out a fulfilling life for myself when younger 애쓰는 그 모습에서 가방만 들고 왔다 갔다 하며: while running back and forth slinging a bag in that striving outlook 시간을 허비하고 있는 내가 : I, who have been wasting the time 얼마나 한심한지 깨닫고야 말았다: Only end up realizing how shocking it is