It is a very difficult situation. We all want to see our parents as loving, strong, sources of guidance. That's our expectation, so it's frustrating when we see them otherwise. But it's important to remember that they are flawed, make mistakes even when they should know better. Your mother sounds like
Obviously I don't know your situation that well, so I'm only speaking from experience with difficult members of my family. You have to take care of yourself, especially your mental health. You can't help your mother, your stepfather, anyone else if you don't have a grasp on your own health. Now, that might mean that you have to put some boundaries in play. If she starts one of her rants, starts making wild accusations, you might have to tell her, "look, I love you, but I'm sorry, I won't listen to this right now." I know you said you don't want to talk to her about your anxiety, but I think if you're very honest with her, maybe that's what you have to do. Explain to her exactly how you feel, tell her your concerns, and if it's too hard to articulate everything in person or on the phone, write a letter/email.
And once you've reached out to her, it's up to her to either stop being selfish or continue to do what she wants. Either way, you've done what you could, it's very important to look out for yourself and get the help you need for your anxiety.
Sorry for what you are going through.