I have the same problem with my games. I lower the difficulty so even I can win it and forget than because of testing and "perfect" knowledge about game mechanisms, I became a true master of my own game. Then even my brother wasn't able to win it and he is pretty skilled, so I know this very well.
You can make difficulty choosable, lower it for casual players but give them option to play "hardcore" or "nightmare" or whatever mode. It can help your game to be playable by more players and still give the most skilled ones opportunity to show their talent.