My girlfriend cheated and now is abusive... help?

So in the beginning of our relationship I found out my girlfriend was flirting with other guys and I chose to forgive her but it's been 16 months now, we have a kid... I find out that she wasn't just flirting but she had sent nudes to this guy in the past. I left her and I met up with an old friend as she was comforting me and we kissed, we talked after. Like a week later I got back together with my girlfriend who found out that I kissed this girl and started playing the victim and calling me a cheater even though we were separated. She would start hitting me and saying what I did was worse than her cheating and if I bring up her cheating again she'll hurt me and my friend. She makes me feel like I deserved to be cheated on... she will go crazy and start being mean one minute than come crying begging for forgiveness the next... I'm so hurt and confused.