I loved single player too.

Th higher difficulties felt so similar to going through Goldeneye on 00 Agent... there's a fine art to going as painfully slow as possible through a level because you know a few bullets kill you. The number of times I'd run into enemy sight, then back around a corner waiting to pounce... :lol
I remember on Dark Agent, during the level with the big bridge, I found it IMPOSSIBLE to go up across it, so I went down underneath into the weird rooms they had there, and camped for literally 30-45 minutes taking everyone out as they ran towards me. Then I came so close to dying on the invisible enemies that had the energy pulse rifle things at the very end. I did it, though.
I'd have that fucking achievement if it wasn't for the Jungle level. I attempted it God knows how many times, watching every single guide possible, and I still couldn't do it. I mean, at the start you basically have to glitch the enemy spawning to even survive. It's
that bad.
When you take the higher difficulties into account I must have played 30/40 hours alone on the single player. Jesus.