Standing up to bullying doesn't make you a bully - although she may perceive it that way. I've been in a similar situation but I've dealt with it by quitting my job and dissociating my way through the working day whilst I work my final couple of months. I'm now at the stage where she can't hurt me anymore, and as Im leaving anyway I don't really care with work. Its sad - as Im sure most bullys are really lovely people underneath the nasty behaviour. But I've spent too long in toxic work environments - some situations can't be changed, and its better just to cut your losses and get out.