Mother's health declining

I don't know what to do. My mother has always been pretty irritable. If I spilled something, if I put something in the wrong place, she would yell at me. It felt like walking on eggshells. At some point, she stopped yelling at me as often. I know she loves and cares me and I love her too, but she sometimes thinks she holds entire authority over the family. She doesn't believe anyone else's opinion and deems her own as "right." But now, all she does is cook, do chores, do work and watch TV on the sofa for over 12 hours a day. I've tried getting her to sit on a proper chair, but she does everything to not sit on one, and her back is starting to hurt because she barely gets up. I tell her it's because she's sitting on a soft surface, but she says "it supports my back", and I know that she knows somewhere that she's lying. I think she's going through some kind of burnout, but I don't know how to get her to get out of it or encourage her to see a therapist. She says seeing a therapist is "stupid." I don't know what to do. Does anyone here have any advice? Thank you.