This kind of is the nature vs nurture debate. Thing is, Nature always finds a way. If it's meant to be, nature always wins. Nature is the ultimate balance act. Look how viruses and bacteria constant grow to new strains as a way of continuing growth. You can't stop nature, hence you wont stop transsexuallity.
I believe though Zoe, that most close loved-ones aren't really concerned with you (effects of so called drugs and medications) but with their perception of how they will be perceived because of your actions.
Like- "What will the neighbours think of me because I have a freak of a child that wants to change gender?!"
Your mother maybe be looking for reasons, to absolve herself of 'bad' parenting that she can tell 'the' neighbours, so the fear doesn't become over whelming. Of course there really isn't any 'the' neighbours- this is just your mother's unconsciousness projecting a fearful outcome of her survival ( ie her place and strength within society ).
Strange isn't it that survival used to be just get shelter, get food and water. Now it's grown into get a job, make lots of money, be successful, keep up with the Joneses.
In light of this theory the best way to have this conversation is to ask your mother what she feels about you as an adult. Let her open up - Ask her: Does she trust your strength and abilities to be a wholesome human being.
If she does, then GIVE her permission to forget any regrets she may feel. I believe this act of letting her remove herself from the fear will actually allow her to see it clearly for the first time, then by human nature she will own it and let it go.
Just my two cents,
Jenny