“네가 필요한데 네게 못 해준 게 맘에 걸려” ("맘을 걸려" in the original is wrong) 네가 필요하다 normally means "I need you" (i.e. 나는 네가 필요하다), so the first translation sounds right for this part. But this meaning doesn't connect with the rest of the sentence very well because it is like "I need you, but I have this regret/guilt in my heart for not having been good to you". If you take the first part as "when/even though you needed (me/something)" as in the second interpretation, 네가 필요한데 becomes a part of "... 네게 못 해준 게" and the sentence sounds very natural, like "I have this regret that I couldn't stand by you when you needed me.". It is however hard to take the phrase this way (although not impossible, especially in song lyrics). In short, both ways seem possible, so you might pick whichever fits the overall context better.