I’ve had misophonia since I was about 12, and I’m 37 now. I’ve discovered in that time that there will be some people that will just never understand it, no matter how hard you try to get them to. My brother is kind of like that. He knows how I am, but he still chews with his mouth open around me. I hate it, but we don’t have meals together very often anymore so I can tolerate it once in a while. I believe he just thought I was overreacting when we were kids, and I don’t think he ever totally got past that image of it.
If your mom is already getting defensive when you try to bring it up, it’s going to be a bit more challenging to get through to her. If you haven’t already, I’d suggest trying to broach it with her in the manner of “this is my condition, and here is some information about it” and direct her to the resources or articles about misophonia, rather than “these are the things YOU do that aggravate me”, does that make sense? She may be feeling attacked, like you’re telling her she’s doing something wrong, when really it’s not her, it’s us and how our brains process these triggers that’s at fault.
For me, I’ve learned a balance of tolerating what I can and excusing myself when I can’t take it anymore is the answer. Even if you can’t get your mom to understand the condition, maybe you can get her to understand that when you reach your breaking point you’ll need to excuse yourself for a while to calm down. Even that bit of understanding might help ease some of the tension. Good luck!