Q: Can you tell my why I can't perform as well under pressure as I do in practice?
A: Pressure is what we create for ourselves, an anxiety about a future event. The more important we value it, the greater the pressure. It's based on fear and self-doubt. Fear of failure interferes with performance and actually creates the problem we fear, the failure. So I hope you can see that fear is the enemy, here. And like most things, from my point of view, awareness is the answer.
If you face a fear, it will eventually fade away. Fear is like a shadow that cannot face the light. If you shine light where a shadow was, it will not be there. The light, in this example, is your awareness, your conscious observation and acceptance.
But you have to have patience. Can you be curious to find out how this process works in your head and mind? It's fascinating how we interfere with ourselves. From what I can see, we're the only species that does this. Cat's don't miss a jump on a mouse (real or toy) and then pout about it. You don't see them looking up in the sky wondering if they're any good, questioning their "catness." They just do it again. But we humans worry about it, think we're "no good," or "unathletic," or "uncoordinated," thus making it worse the next time. Our minds actually interfere with our ability to grow and perform.
If you can observe the process, and find a way to overcome it (actually it will be overcome in time just from the awareness and curiosity about it), you will have learned a huge life lesson. It happens to all of us. The inability to perform on a golf course what can be done so easily (or relatively easily) on a practice driving range happens to EVERYONE in the game of golf. It's the subject of countless articles and books. You're not alone, but if you can learn something about this process of interference through basketball, it will help your entire life.