How to get past a mental block about coming out to my Mum for the second time

Okay so, I am currently out to my mum as nonbinary, i have been for about 4 maybe 5 years. Two or three years ago I tried to come out to her as trans and it didn't go great. She reacted in a 'I will try and will at least be respectful as i love you' way but then immediately barraged me with reductionist statements combating the idea of gender and trying to invalidate my identity by reducing it to clothes, makeup and posturing among other things and tried to tell me I could do all that as a guy.

Anyway, she really fucked with my confidence and within a day I'd retracted the coming out and had gone back to identifying as nonbinary to her (she's the only family member who knows anything) and I did the same with all the friends I'd told hiding behind what my brain believed was a safer identity while slowly over the course of a year building my confidence back up with it and telling my friends again once I felt safe to do so.

I am now at a point where for anything to actually change in my life I need to tell my family. But I don't want to rush that as the majority of them have expressed bigoted views in the past including my sister and her partner, aunt, uncle and dad. In June I have to leave my uni accommodation and move back in with my Mum, she's more reasonable.

I want to tell her so I don't have to hide all my stuff and purge things. I want the chance to be comfortable in that home once I'm back there as its just us living there for the first time ever.

Problem is because of the reaction I received the last time my anxiety condition and I guess some of my fear of rejection and autistic traits are all coming together and creating a sort of mental block. Anytime I've tried to talk to her even when we've sat down in the evenings (I go home once every two weeks) I've just not managed to get any words out. I'm utterly terrified and just don't know where to start in bringing it back up. It has to be in person, I can't text her because my sister and niece use my Mum's phone frequently and it wouldn't be safe for it to come out that way.

So I need some advice on what to say and how to start the conversation.

I really hope you guys can help. This is the next step. I won't be telling the whole family for a while afterwards and so won't be able to immediately come out or anything, but this is the next step.

Thanks sorry for the all the rambling