Usually a letter or an email works best as you can get your thoughts down and be concise as they will be in shock I suspect. Allow for that and the Kubler Ross stages of grief Shock, Denial Anger, Bargaining, Depression and Acceptance. Although they dont always follow that clear pattern they can jump about a bit.
As they will likely go through them even if they are not saying so. As this is a form of 'loss'. But also rebirth a new cycle and new start.
If you are not in regular contact then if they get angry and turn around and walk away then nothing much has changed. This is about them now. They might need some support too? So be ready for questions.
Keep us updated you can always post here for feedback.
So in conclusion get on your PC or Laptop ot tablet and write a letter and explain but keep it as short as you can as it will likely be overwhelming. Most people these days unless they have been living under a rock in a cave will have soe grasp of transgender people. So it might go more smoothly than youd think. You could at least have girly chats now?
Ice Maiden
