To borrow and paraphrase from Peter Svidler, here's a vintage classic:
I ended up in a passive position. All my pieces are behind my pawns and don't really have a good way to get out and contend the game. I felt terribly demoralized and more or less just gave up. Analyzing with a computer afterwards on ICC, I realized that as passive as my position is, White is not really winning by force for most of the game.
So how did it go all wrong?
In this position, I chose to capture with the knight instead of the bishop. The computer rates both captures about the same but I think I'm more comfortable defending the position with the
knight on f6 instead of the bishop on b7.
After the exchange my knight on d4 can be trapped in some lines. The only square I can move back to is f5. I decided to move the piece back out of a potential trap and subsequently de-centralized my only active piece. I should have analyzed more concretely. There are lines where the piece can be trapped but I should force concessions from White before placing the piece back passively. On move 15, there was really no good reason to play Ne7. If White kicks the knight with g4, then he has loosened his kingside and provided some opportunity for counterplay. By moving back to e7 voluntarily, I have handed over way too much for far too little.
So once I got into a passive position, I started playing actively. It's counter-intuitive but I think passive positions must be played passively. Just play to create a rock hard shell that is tough to crack. So the pawn moves 16... f5 and 17... a6 are probably not quite in the spirit of the position. I'm worse, but let White figure out how to win.