Change is scary. Transition, your Mom buying you things that don't make sense to your life till now...It is all change and change is scary.
I don't know why I think I am female or in your words, "want to be a girl". It just is in me. I am not wholly at peace with it yet. I have only been dealing with this thought for about a year now. However, I can tell you that the urge to dress as a girl and do things as a girl has been with me my whole life. I am a lot older than you. It doesn't go away. So, while you may not understand why, you may still want to embrace the idea.
So much of life is ahead of you. I know at a younger age, I would be frightened that I am going to screw up my whole life. It doesn't make it any easier, but I can tell you, life can be what you make of it.
Good luck. Keep reaching out to friends, counselors and this site as often as you need.
With warmth,
Joanna