I would like to start with this question, because your answer will predict the result of your performance.
Here is why I was able most of the times to play at my best on stage and better than alone or in front of my teachers. For me, the stage is an inspiring and lovely place where I like to be. I can feel the energy of so many people and their expectation to receive good and nice feelings from me, the artist. That's great and it should be like this. I have something to offer and there is somebody ready to receive it. That's a very good match. When I was alone or in front of my teachers there was not such an atmosphere or such a need, because we were looking for mistakes and for ways of improvement, so that the performance would be a very good one.
As you can see, practicing and playing in front of your teacher are different experiences than playing on stage for an audiece. You need to switch mentally and emotionally from one experience to the other. Next time when you'll go on stage, think about it: What does the stage mean to me?
In my book Coaching for Musicians, you can find two surveys which will help you asses your level of stage fright and performance anxiety and some useful suggestions to overcome them.
If you are ready for a change, you can contact me and together we can find an answer which motivates and inspires you, so that you'll be able to play at your highest level when it matters most.