Know why Stephen Curry can bomb in 3 pointers like it was nuthin' no matter the stress? Because he has practiced 3 pointers from everywhere so many times that from the time the ball is in his hands and his eyes see the hoop it is all automatic. Muscle memory is developed through practice. The more you can rely on muscle memory the more confidence you will have regardless of the pressure.
So, when you miss a shot while practicing, set it up again and again until that shot is automatic, practice some more, then come back to the problem shot and make it a few more times. Success breeds success.