The most important thing to do with any blunder is to stay calm. Evaluate the position objectively: pretend you've just been handed a chess board and told to finish the game.
In this position, you've got a well-defended e-pawn and a weakened kingside, especially
g3. Your King Knight's usual best spot is unavailable. However, the position is certainly not lost: there are many perfectly good opening lines that feature an early
f3 from White. Taking some inspiration from them, you I might be inclined to go for a setup with
d4 (exd4 Nxd4), and
Nc3 to establish a strong center and dominance over the light squares. An example line might be:
Resulting in a position that looks more like a line from a Knights' game than one with
[FEN "r1bqkb1r/ppp2ppp/2np1n2/8/3NP3/2N2P2/PPP3PP/R1BQKB1R w KQkq - 0 1"]
I can't overstate enough, though, that the most important thing is to just evaluate the position objectively and try to set your ego aside when it comes to past moves, especially mistakes.