I want to stop having contact with my mother.
Why don't you want to have any contact with your mother?
Our relationship is very toxic, she's done some terrible things to me.
It sounds like your relationship is very difficult. Try talking to a therapist about it and see if you can repair the relationship. It seems difficult but it may not be impossible to fix.
No, it's beyond repair, it's over.
If you really don't think you can have a relationship with her, then you have to put your happiness first and cut off contact. You deserve to spend time with people that love and care for you.