I Would like to know your age first. Although i am not doctor, (i also had this problem it is known as Puberphonia
), you might want to know how i treated it.
If you are able to anything any sound in a low tone(sometimes cough
or laughter may be in a very low pitch). Then you should practice saying "aaaa" or "hmmmm" for a few seconds and then should try to read something out in that low pitch, you might not be able to speak properly at first or stutter a bit but after you starting reading it out without a stopping(stop only to take a deep breath and continue reading) then you notice that you can speak in a low pitch voice effortlessly. Most Speech Therapist's try to bring out the low pitch voice and then stabilize it.I think doing this for a few weeks you will become so accustomed to this voice that you will not be able to speak in your earlier 'girly' voice.You may feel very bad after speaking in the low pitch voice for a few minutes you are not able to speak in that voice anymore but continue in your 'girly' voice.
Remember doctors may say that it can damage your vocal cords(so ask a doctor
) , but i was too frustrated to listen to any(when nobody tries to listen to me) so i tried it out.
Here are is a link which should prove to be helpfull to you:-