Toxic Relationship?

I am 33 years old now, but I think I am still suffering the consequences of the toxic relationship between my mother and I. I have read the definitions of a "toxic" relationship and she falls in that category. Now as an adult, I still have a deep feeling of guilt if I don't call her back or respond to her texts and when I see her, I just feel extreme anxiety. On holidays, I feel like having a panic attack because I cannot stand being around her for an extended period of time. My little brother just loves our mother because she babied him, but I was the tortured one. So, how does a person overcome this? I am an adult, but still afraid to stand up to my 64 year old mother! Sounds funny, but .. she's mean. :(