I wish I could take it back, I wish I could take it back
But it's too late
I wish I could take it back, I wish I could take it back
But it's too late

from Proof – Kurt Kobain Lyrics on Rap Genius

Meaning

The songs hook, the most powerful part of the song.
A deep breathing sound can be heard at this part.
This could be 3 things.

1) Smoking Pot or just a cigarette to calm his nerves before he takes his life.
2) Inhaling some kind of poison.
3) He might also be taking a deep breath, when people find themselves in stressful situations (in this case he might be thinking of killing himself) they tend to take deep breaths to calm down

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