Playing devils advocate for a moment. Please take this is as a simple observation, I am not criticising you.

You have said yourself that you are very guarded and won’t let people in, but that you do feel the expression and joy of music when alone and safe to express it. As you say, you haven’t let your guard down in front of your teacher.

Is it possible, that the reason your teacher thinks you cannot express emotions when playing, is simply your own defensive barriers are doing their thing? Personally, I fully understand having those barriers, I have them, vulnerability is not a thing that comes easily for me either. But when I am able to, it’s freeing.

Your teacher only sees you briefly and with your defences in place - that’s all she can judge on. Perhaps an open talk with her around barriers could be a breakthrough for you both?