Hi S.,
It sounds like your daughter is trying to 'control' her situation or her little world because she lacks control in other areas. She is frustrated and is acting out.
My mom was a single-mom who dated quite a bit and we moved around a lot too. When things really spun out of control, I began to scratch others terribly at school, usually boys, making them bleed and I would get into fights all the time. I was normally a very loving, sweet kid, but I freaked out. I had so much rage...but my mom never beat me or hit me or anything like that.
There is a good chance that your daughter is trying to voice her opinion about your engagement and all the changes in her life and cannot get it out.
Sit down and talk with her about how she feels regarding all the changes in her world. Just let her talk and make sure she feels safe regardless of what she tells you. Do not punish her for saying things like "I hate the guy you are marrying" or "I hate you". She does not mean it. She just is too immature to voice her concerns in a loving way. Reassure her that regardless of what she says to you that you love her and you will do whatever it takes to help her with all these transitions.
Get her into see a counselor that specializes in working with young children!!!!
I promise, you will see results and grow closer to your daughter in the process.
:-)
Best of luck,
S.