Ok so I have some really good headphones and I thought I had good ears, up until now I can't tell the difference between quality anymore.
I swear I can't tell the difference between a 128kbps mp3 and a cd rip.
What the hell is wrong with me?
Maybe my ears are just getting bad or what.
Good headphones, you say. What about your sound card? A cheap, crappy sound card can be an even worse quality-eating fiend than cheap, crappy speakers.
Also worth considering is how advanced encoder technology has become. If the 128 kbps MP3 was made with a recent version of LAME, depending on the kind of music (and how tired your ears were at the time of the test), it just might be expected to be near-transparent. Check this listening test: http://www.listening-tests.info/mf-128-1/results.htm Of course, that is not a reason to use 128 kbps; disk space is cheap and you have more assurance of quality with a VBR preset like V2 or V0. Also, there are some songs that will definitely not sound the same at 128 kbps, even to you (I'm thinking of Autechre's "Second Bad Vilbel", but that's just an extreme example).
If the 128 kbps MP3 is made with any encoder other than a recent version of LAME, then yes, there probably is something wrong with your ears. I have 128k MP3s I encoded with MPEG Suite seven years ago, and they're unbearable. (But I had a crappy sound card at the time, and wasn't a careful listener, so remained clueless.)
BTW, while I don't blame you for posting this here, I think that questions of this sort would be more suited for /tech/ in the future. Lossy audio coding technology != Music. Not a complaint, just a suggestion.