When lapses of concentration cost dearly

Instructive lessons from this game: (1) when playing turn-based chess, don't forget your plan in between moves just because you didn't look at the game for a day or two (2) when you do end up making a mistake, don't make rash decisions immediately afterwards.

The story here in the opening was all about competing to get knights advanced onto weak squares - my knight to d4, my opponent's to d5. I managed to succeed and beat my opponent away on account of his d4 square being weak, and my d5 square only having an appearance of weakness, when in reality I was able to defend it.

But then, when I can only assume I was playing while sleeping, I instantly weakened the d4 square, and the white knight hopped right back in on the action. I was so miffed that I completely missed the simplest knight fork, and dropped my queen the next move. Here it is: