"Mom, I'm having a hard time telling you this, but it's something that has to be said. You might have heard of transgender people. They're people who are born with a need to be a different gender than their body sex.
Well I need to tell you that I'm transgender, and I won't ever be happy unless I can live as the woman I am.
There are several things I really want you to understand:
1. This isn't something I chose or that I can help. I was born transgender, and there's no way to change that.
2. Transgender people who can't transition experience gender dysphoria, which is intense unhappiness about their gender. It can cause anxiety and depression, and it's often impossible to be happy. You don't want that for me, do you?
3. Transgender people who do transition to live as their true gender have a very high rate of success.
I hope you'll understand that this is something I have to do. I know you love me and would not like to see me live a difficult or unhappy life, so I hope I can count on your support.
Thank you for listening."
Do you think something like that would work?