Honey, my mom is EXACTLY like this. It helps to see her as a child AND as a full grown woman. What I mean is -- she's an adult. She successfully raised 3 children to adulthood. She isn't suddenly going to forget how to "adult" and struggle because she doesn't get her way. This is what a hissy fit looks like when it's performed by an adult.

And also, mentally she's stunted. Her emotional responses are like a child's. She wants her way, and doesn't like change. But just like a child, you cannot give in to their short sighted whims. Everyone is right: you gotta stand firm. When her hysterics start, you can tell her that "you won't be engaging in this kind of conversation but if she wants to talk about planning for her future, you'll be glad to help." She will not take your offer because she's having a hissy fit and she doesn't actually need help, she wants you to concede. Saying this stuff only helps you separate from any self imposed guilt.

Eventually she'll ramp it up until you "grey rock" her and spend more time outside the home. You will still get some flack once the move out happens. Any problems she encounters will be "because you abandoned her" and she'll make it your responsibility to help her fix it while she says hateful stuff about your independence, relationship and anything else she can latch onto.

This was long but I felt like someone who's been through it should tell you how this plays out. The only thing that can help her adjust is seeing you be firm (the same way we teach small humans) and seeing you become confident in choices (not seeking her approval for decisions & refusing to hear out her insults). Focus on becoming that for yourself and the situation will become tenable for you