How can puberphonia treated?

I am a 25 years old man having voice problem for the last 10 years. I speak in two voices – low pitch and high pitch voice. The doctor told me that I am having puberphonia and suggested speech therapy for me. I have taken 4-5 sessions of it. The pathologist is able to produce my new deep voice but I am still unable to adapt it when I speak or start speaking. I forget how to speak with my low pitch voice and use my high pitch voice. Please suggest me any drugs or surgery to treat this.