What is the redacted lyric in Portal 2's credits?

The lyric is "I'll stop feeling so bad". The reason it is redacted in the first place, is because GLaDOS is, in her own way, admitting she has grown fond of Chell. Redacting it prevents that admission of momentary weakness.

If she was happy Chell was gone, there would have been no need to redact the lyric.

I think this song is captioned if you've turned on subtitles, which will confirm the above.

I actually believe the following:

GLaDOS says "I'll stop feeling so GLaD." The captions say "I'll stop feeling so bad." The monitor tells us [REDACTED]

This is only to mess around with the player, obviously GLaDOS doesn't want to show her weakness of feeling bad for what she did and tells the player she is GLaD instead of feeling bad. However, that's just her version of the song, the official lyrics are "I'll stop feeling so bad".

That's probably the joke of the song, that line of the song is shown in 3 different ways to the player, all meaning a totally different thing. It wasn't "REDACTED" for no reason of course...