I laughed when I read my comment again , I mean nominalisation and not nationalisation,. I know thatㄴ/는 modifies nouns,thanks for explaining that, but the problem is when I see ㄴ/는+걸 and also 을+걸 at the end and the sentence is finished with it , - thanks for your comment again.

-ㄴ/는 is very common in Korean, not just in songs, but used everywhere. It is what you add to a verb stem to make in an adjective so it can modify the noun 것 or any other noun. In your example, 아름다운 것, 아름답다 is the adjective "to be beautiful", and it becomes 아름다우 + -ㄴ to make the adjective "beautiful". And then you can put the noun 것, 걸, or any other noun after it. Same thing with 어쩔 수 없는 걸. The verb 없다 changes to 없는 so that it can modify 걸. Hope that helps.

thank you Mojave for commenting, I think ㄴ는걸 is an ending that comes at the end and have a special meaning,I think it's the nationalisation ㄴ는것 plus을 - I hear it almost every day in songs but I really can't get the meaning, -thanks for reminding me about 걸 that means girl, they pronounce it right (geurl) in songs and in most cases I can differentiate it.

걸 could be "girl" or "thing", which is 것 with the object particle 을 = 것을 or 걸 in contracted form. It depends on the context of the rest of the lyrics