in once more with feeling where duse buffy say she is liveing

"I live in Hell, 'cause I was expelled from Heaven."

That the line to which you're referring? She says it in "Give Me Something to Sing About."

I don't she does, but I think this is maybe what you mean-
Life's not a song.
Life isn't bliss, life is just this, it's living.
You'll get along,
The pain that you feel, you only can heal by living.
You have to go on living.
So one of us is living.
Spike singing to Buffy
it's towards the end it's apart of the song Something to Sing About
I hope this was helpful, sorry if it isn't what you were thinking of! :)