Think you are good? Challenge yourself!
I thought the game was pretty hard, but you eventually get used to it and I memorized the levels pretty fast. I found a way to challenge myself without the need for new maps.
If you would like to join me in my quest:
Console (~)
type: "DebugSystemUI" and press enter
the debug menu should appear.
go through the tabs at the top and in one of them should be: "Mat_Fullbright 0" or something close to that.
Change "Mat_Fullbright" to "Mat_fullbright 2.00"
this changes the textures so it is harder to tell which surfaces can be portalized.
some options in the debug menu to avoid:
Software Lighting
World
VGUI
anyone else have any console commands or something to challenge yourself more? 
Turn the monitor off?
Turn the monitor upside down?
Story: 22.22222% Design: 33.33333%
Levels: 4.44444% Cake: 0.00000%
Voices: 0.0^3% in base 4
New models & Textures: ((e^28/pi*7)/0 - 42)%
Grief counseling: 111.11111%
long i like the upside down idea. But I get the feeling your being sarcastic 
Was it the that gave it away?
Anyway it was only because I don't know any console commands. It's a good idea.
Story: 22.22222% Design: 33.33333%
Levels: 4.44444% Cake: 0.00000%
Voices: 0.0^3% in base 4
New models & Textures: ((e^28/pi*7)/0 - 42)%
Grief counseling: 111.11111%
Turn down your graphics all the way and turn off Portal rendering.
You can't see through Portals and then you can't read signs either.
Personally I think that, if I remember correctly.. only a few maps of the original 19 were actually somewhat hard. But for some reason I cannot seem to unlock the additional maps.. its like.. I dunno, I want to play these advanced maps to see if they are more challenging.
cl_portalgun_beam_size 100
Good luck.
I was glad to see it go. The damn thing told me it was gonna shank me while I slept.
I think it looks like my uncle.... 