“We have been separated for 5 months. My husband has moved on with his life and living with his girlfriend who my kids (twelve and eight) get along with. I have met a friend with whom I have fallen in love with. My children don’t seem to adjust very well to this. I have spoken to them about the relationship but my daughter (12) always seems aggressive. She has tried to stab herself and pack her bags. She calls her dad and makes me a bad mother. She always says that I have no time for them since her dad has left. She threatens that she will go and live her dad.”

It's NOT the child...
You are surely not surprised? There is no easy solution. The problem is NOT your daughter.
If I were writing the script (which I am not) I’d have both adults resist finding “love” until they have allowed the children time to get somewhat used to the first trauma (mom and dad’s divorce) before I thrust them into the next (mom and dad’s new relationships).