How old do i have to be to say where i want to live?

i was previously in foster care. i am not 17 turning 18 in about 4 months, i have a job and i i am going into my senior year of high school. My foster parent has given me permission to reunite with my mother and live there as long as i kept in contact with her and let her handle legal buisness. well i do not wish to seek her in my life anymore, and i am getting threatened by her that she will call the cops and make me go back with her. my grandmother says she isn't going to let that happen. how far can this go? and do i have any say on where i want to stay even though my foster parent was given custody of me?