There is only now

The way to be is to be, I am full with wise wise phrases... I know a few tricks in chess, but much too often I lose concentration in vital phases of the game. We all know one bad move negates 40 good ones, how do you keep concentration?

I have tried many ways to guarantee my chess-mind is present, like doing some puzzles before playing. Doing some chess lessons before I start playing, or playing some blitz games to check whether I am sharp. Because I noticed that there simply are days, or hours, where I look at the board and miss half the possibilities ... but also that I see 90 or even 95%. It makes it extra frustrating to give a game a way simply by missing a bishop hiding between the pawns or such.

The worst I ever had was: My opponent did a total blunder, moved his queen in the sight of my queen undefended, and I so much did not expect that that I missed it and he took my undefended queen with his... I mean, wow!

Do you have any tricks or a system how you keep focus, how you avoid blunders?
Or is it more rare than I expect to have such trouble focusing on the game...