Maxx,
At 15 years of age, we all experienced thoughts of self doubt and confusion just like you do. You mentioned a therapist. Is he or she experienced in transgender therapy? You said that your father is accepting. That is very good, since many of us get disowned and thrown out of the house. Maybe your mother will come around later. Having the support of both your parents is very important for everyone your age.
In a few years, you will be on your own. That is when many of us take our first steps to transition. Some of us wait until much later and try to tough it out. That is what I did and I just had to finally be myself.
Currently the FTM surgical procedures are limited and only partially create a fully functional penis. Maybe the future holds some improvement in this area.