Who said anything about enjoyment? Try Penumbra, or Amnesia.
I'd also highly recomend The Orange box. TF2 alone is worth it's weight in hats. the half-lifes will give you 30 hours of solid gameplay. There are quite a few fun TF2 mods out there to toy with.
Garry's mod obviously.
I know this might seem like crazy but, have you considered not gaming? I mean, you are meant to have fun, but if you're not, try taking up another hobby. Try knitting socks.
My seniority means you should probably just agree with everything I say
When I get apathy I hit up acrade/casual games like
Bit Trip
Osmos
Plants vs Zombies
Suggesting some of my favourite games, also they are pretty cheap on Steam;
Zeno Clash
Mini Ninjas
The Path
Also I just play with friends, because ultimately gaming is fun because you play with fun people, in my opinion;
Diablo 2
Warcraft 3
Alien Swarm (Great because it is free so there is no excuse for not having it!)
Starcraft 2
Dawn of War 2
Same. I've played multiple characters because there's a lot of variation between each, so it's like the game is new again. There's also a few dlc's which extend the game further.
Yes,played trough the whole game co-op with 2 of my friends.Strongly recommend this,the most fun you will have in a while.