Mental readiness

There's this small matter of mental alertness. If you don't have it, how do you get it especially during the middle of a game when you're nervous and your mind is racing through a million thoughts not about the matter at hand. You manage to engage either your arms but not your legs, or you blank out entirely and swing blindly late at some flash of a ball. Those are my symptoms. There are dozens of things I have to do right in order to play well and during this lapse of concentration, any number of those things will get forgotten. I could forget to square off, bend low, watch the ball, cock my wrist and elbow, get on my toes, breath, choke up, and so on. I have tried to remember them all, but hardly ever get them all right. What I need is to concentrate on just one thing that can bring all of them into play.