Have you tried talking to your mother? Granted, she might go off on you, but getting her to see things from your perspective might help.  Maybe she doesn't know just how controlling she is.  

I'm no doctor, but it does sound like something might be medically wrong here. Disorders such as bipolar disorder or manic depression can cause extreme outbursts, such as those you describe your mother as having.  If she refuses to go there sadly isn't much you can do to force her.  Obviously, she's an adult and makes her own decisions-- wrong or not.  

In the meantime talk to your step dad about this. You and him might be able to get across to your mother, together.  Talk to other family members-- anyone who might be able to help.

I'm sorry I can't give you any better advice than this, but honestly ,I'm not sure what else you can do.  Hopefully there will be others on this site who can advise you better. Either way, I wish you the best of luck, and get the help you and your mom needs.

Best Wishes,

Sam

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~LyricalAffair is my bff~