... I'm going to be honest, I've ever only found two secret areas in the entire first Doom game. Some games (read: all games) are just too hard for me.

...

Ouch. Turn down the difficulty level? :P
Also, mouselook helps greatly when you're playing Doom. Like, seriously. A lot. Along with strafing. Mouselook + strafing = Easier gameplay. I'm usually the kind of guy who plays on the easiest difficulty, but i can manage the medium difficulty in Doom II due to mouse and strafing. Secrets in Doom are diabolically hard to find, so don't feel too bad about it. =P