I'm sorry to hear there will be changes in your family. If I understand the question correctly, you are wondering how a separation or divorce will effect your child?
She will be confused, upset, sad and angry. But, if the marriage is unhealthy, and you and your mate have done everything you can to make it work- and it's not working- then she will survive.
It's important to not involve your daughter in adult conversations or decisions. Remind her how much you love her and how that will never change. Always speak positively about her other parent, even if you disagree with that parent. You can say, "I'm not sure why your mom made that particular decision, but I do know that she loves you."
You can make it through this transition. Stay available to your daughter, in case she wants to ask questions or get reassured of your love and some stability in her life. For more help, try The Blended and Step Family Resource Center- it has tons of articles and newsletters that address these issues.
Shirley Cress Dudley, MA LPC NCC
Blended family and single family expert
Director of The Blended and Step Family Resource Center