Willempie,
I came to the same conclusion that you have about trying to be too theoretical. When i play strong players who play into my "theory" I just can't help myself; I just have to play the line I think is best.
Therein lies the heart of my problem: I like to play the "best" lines, even if they don't give the best practical results.
One of my most common opponents (a national master) plays 1.e4 c5 2.Nh3 (or 1...e6 2.Nh3) and still manages to win. His opponents, myself included, get the better position out of the opening, but he is able to outplay us in the middlegame because he has specific attacking plans that he has worked out at home.
So against this player, I tend to lose from "better" positions. There's really no point in spending hours upon hours working out how to beat the
opening in cases like this, but I still tend to blame the opening rather than the resulting middlegame positions.
I can outplay opponents of similar strength if they don't respect the opening, but the higher level players tend to reach a theoretically inferior position and still outplay me. My favorite variations aren't really letting me down so much as I am letting them down.
That's depressing.