Chess opening: learning the hard way

I spend a great deal of time memorizing long opening openings for my chess game. But there are times that during one of the matches I played, especially in club or below master levels, deep theory of complex openings such as the Sicilian and various Indian variations, no matter how scientific they are, are not always that easy to put it into practice or have it execute the full potential of the specific openings in this levels. Maybe there were moments in one of your matches that you claimed you are an "expert" in memorizing say the Nimzo-Indian Defense variants all the way up to 15 moves or so and you are feeling comfortable that you're opponent is falling to your trap but then suddenly your opponent, whom you keep crossing your finger all through out the opening will stick with the theory, but suddenly divert the flow of your opening into something "new". Isn't that the most frustrating thing that can happen to a chess player especially if you are a die hard fan of a particular openings and variations that only spring up in about one per ten matches you've played?

I have always though that the most depressing opening for me in my reply's to white's 1.d4 goes something like this: 1...Nf6 2.c4 e6 3. Nf3...oh no this can't be! He's denying me the opening theory that I spend a great deal of time memorizing but what the look what this fellow had done. I have to admit that Nimzo Indian has always been my favorite reply against whites 1.d4. But since I can no longer always hope that white would always give me the option of playing that opening, He left me no choice but to push 3..a6, the Queen's Indian, an opening that I'm quite insecure to play about. But anyway, memorizing openings without understanding how each moves work, especially for the more inexperienced player is generally a waste of time. It is good to see what the opening options are, but sticking with the same opening and ignoring the play on the board is a mistake. I learned it the hard way.