You are 15? Well, then I would not bother at all about "not being good enough for your age". That is, if you want to become a professional, you have ALL the chances. Never mind that progidy myth, it is just this - a myth, kept alive by those who want to protect their own interest. You can be and become everything you want. Period. Never let any idiot tell you anything else.
I think it is very encouraging and exciting to have more than teacher. This past summer I spent a week at Chetham's piano summer school in Manchester, UK, and it was unbelievably nice. I am not an upcoming young concert pianist star, I am 48 and just a happy amateur who plays for pleasure and I'm not very advanced either. The mixture of students at the piano school was most stimulating. The youngest were 9 or so, the oldest close to 80. The faculty consisted of renowned piano stars like Noriko Ogawa and Leslie Howard, and all they asked from you was to love the piano. I met people who were planning for their next CD release and I met a girl who had just been playing for 6 months, and everything inbetween. So I just enjoyed the atmosphere and learned A LOT, of course. And of course I will go back ... I have also enrolled for another piano summer school here in my home country this summer.
And here, back home, I have my own teacher who I met 2 years ago. We get along very well and she is constantly encouraging me and thinks it is very exciting that I do this other stuff in parallel to her lessons. I also know a concert pianist who is willing to give me a few lessons. And this is not because I think I will become the next Argerich next year, but because I like to play the piano.
I think you should happily continue with your lessons with your regular teacher, and also most happily add other courses and lessons with other teachers if you like. Nobody has the right to tell you that you are not "good enough", or what your skill level "ought to be" right now. Why your teacher is not encouraging you right now, I don't know. Maybe she thinks you should concentrate on school at this moment, or whatever - well, never mind, do what YOU want to do. It is your life, your time.
Besides, remember that even if you were the most promising pianist of your time, there would still be people saying that you are not very good or even mediocre. It happens to everyone.