Being bitter?

Why is it that I’m being called bitter because I do not want my child around the female my ex has be cheating on me with? She wished harm upon me and told me she wished my child wasn’t around so it could be just her and my ex. How my child is a burden etc etc. My daughter says she’s not nice and screams at her when she’s there. Mind you they’ve only been together 6 months. I brought this all to his attention and he doesn’t budge and says I’m bitter! My 4 year old doesn’t even want to go to dad’s house anymore. Why am I being called the bitter one here?