Fighting off fear and failing

Theres a lot, and Ive never posted like this before, but I feel like I should and that it might help.

Tiny bit of background on me, I have generalized anxiety disorder, among some other things. My mother has undiagnosed issues and multiple head injuries and used to be emotionally manipulative and abusive and thats affected me in different parts of the friendships and relationships I have now. My mother is doing better, but is not perfect.

Last week I had a big fight with my roommate. My roommate has an alpha personality. Shes always right, in her eyes. She told me she would help me with something, and then towards the end ditched me and told me not to talk to her for a while. We had a confrontation later, where I did my best to remain calm and only yelled once, and it turns out she felt I wasnt being grateful. That my thank yous and I appreciate what youre doings were guilty ones. Technically this wasnt really resolved but we ended it with saying were both good.

but I need to move out earlier than expected. I have to tell her this, and tell her that I cant pay for Julys rent because I wont be living there and my parents wont help. And Im terrified, but I know the conversation has to happen. I just dont want to feel afraid during it, but I know I will. I dont know how to prepare. I just know Im going to mess it up and I feel like crying.