As Ayden said, this isn't uncommon. It's the bargaining phase of grief: your mother is resistant to the change you need in your life, and she's using whatever she has to try to stop you so that things can remain the way she wants them to be.
The thing is: your mother will not die if you transition. She will continue to live. She'll have to go through a difficult period of adjustment, but she'll get through it. And in time, she will come to accept you for who you are... and when you're happy and well-adjusted in your own skin, she will come to realise that you've made the right decision for yourself. And she will realise that your transitioning was a heck of a lot better than you dying, because she still has you in her life and you're finally happy.
My children said they'd never want to be seen in public with me if I transitioned because I'd be a 'freak'. Well, I take them out in public all the time. And they still call me 'Mum', even with my beardy face and baritone voice.
People adjust. But they don't want to have to adjust. That's just too bad: everyone has the right to be themselves, and you don't have to be forced to suffer for the rest of your life just so that they can save themselves the trouble of adjusting.