I just want to say that a negative reaction like this never is about you, it's about them. Their insecurities, their ego, their discomfort with having their worldview challenged, their hangups, etc. Don't take anything your mom does or says right now personally.
The first place a mother's mind usually goes is to blame themselves for your turning out this way, and often they don't want to accept the blame so they will project it off onto anything that's handy, or they will just try to deny your struggle is even real. In a weird way, this is coming from a loving place. It's crazy the horrors that can be born from twisting love with human failings. But part of why she is so resistant to any of this is she is afraid for you. She worries that your life will be harder if you pursue transition, that the world will become a dangerous place, that you will be cursed to struggle for success in a cruel world for the rest of your life and that it will be hard (impossible, even) for you to ever find love. All of these things are running through her mind, I'm sure, and she wants to protect you. She is just flailing, trying to find a way to keep you from having a hard life.
Also idk how much of her ego is wrapped up in you, but it is something that you sometimes encounter with family members. How I mean that is how does she feel your being trans reflects on herself? Is she afraid people will look at her and judge her a certain way because of what you are going through? This is something, personally, that was more of a dad issue than a mom issue for me, but I think my mom had a little bit of that going on, along with my siblings too, tbh.
I don't know, there is just so much that could be happening in her head right now. Just be aware that most/all of the terrible things people say initially are just outward, visible flashes of how they are coping with the rush of feelings that comes with a revelation like this.
It may be a while before things get better. Your loved ones are going to have to go through a sort of grieving process for the son/brother they thought they had, and that can be so rough because you will feel responsible for their sadness, and you'll want to say or do anything to soften it. Whatever impulses you have in that way, don't cave, be firm, but also be loving, supportive, and kind.
The best thing you can possibly do to help your loved ones get behind you is to simply have a successful transition. Prove all their worries false. Start to show how living authentically improves your life. Be happier, kinder, more loving, more fun to be with. Make friends, fall in love if that's something you want. Be glowing, be the very best version of yourself. You can't always be those things, not everyday, I know that all to well, but when your life gets better and you become a better person because of your transition, those things will go a long way toward helping your loved ones get on board. And you don't have to fake them (you shouldn't fake them, really) they really do just come with transition automatically when it is the right thing for you. They come with time though, don't expect amazing changes to your demeanor and outlook on life immediately, just like you shouldn't expect the physical changes to happen overnight.
Hang in there honey. It does get better.