I guess you may need to get to the heart of what has you terrified. Rejection? Being cut off? Anger? Ridicule? Non acceptance? All these things are possibilities. But like many fears they are often bigger than they seem, and usually never as bad as imagined. Still you do need to prepare for the worst, as the saying goes. I understand the fear, I haven't told my family yet.
I did tell my mother the first time I transitioned. I chose to tell her face to face. She's not a scary person but I was petrified. She didn't take it very well. But at least by telling her directly I was able to answer her concerns and questions (and she had plenty) and respond to her denial (apparently because I didn't "act gay" it couldn't possibly be true...?!). Also I knew straight away where I stood with her (that I was still welcome to visit as long as I didn't come as a girl). The thought of writing a letter never occurred to me, I wish I had but not to give to her, just so that I at least had everything straight in my brain before I told her.
Anyway, you've written it, but you don't need to give it to her today, tomorrow or even next week. Don't tear it up. Maybe have a think about other things you might need to tell her to deal with those fears you have? Rewrite it in a week and see if you can send it then. If not hold on to it for another week, etc. in the meantime, is there anyone you can talk to about your fears and concerns. I'd suggest you don't tell your mother until you feel safe about doing so.
Hugs
Grace
