It would be interesting to know if there is any such study - I suspect that there have been too few under 16 teens treated with HRT over the last ten years or so to really establish a viable long term study. That may be starting to change of course, but then the question is how long do you wait to see before you decide they haven't changed their mind. What you need to do is find some trans people who were placed on HRT at your age and, some 10 or so years later, are happy with their life. Perhaps your therapist can help in that regard.
The other issue is whether you could be treated with HRT, some local laws won't allow anyone under 18 or 16 to be treated with HRT even with parental approval. Again, your therapist should know the legality of that.
Hang in there, I know it seems unbearable right now, but they may still come around sooner rather than later.