How can I get better in game?

can someone give me some tips on how get Better in game? When i 'm doing the workout I'm like a beast but when i'm in game I'm lite rally useless and I forget what i've done in the previous workout. I get anxious and in my mind comes questione like:and If I miss the shot,and If I get a foul etc. can someone give me tips(Sorry if I got some words wrong,i'm not english or american)