I think she is old enough to explain to her no matter what she is always going to be the most important in your life. But that the same way she has friends in school, you need friends too. Try asking her about her feelings and why it is bothering her so much.
Does she see her father?
I am sorry, I have no idea what to do, other than recommend my parenting counselor, the author of Raising Our Children, Raising Ourselves. Some parenting issues cannot be solved by sharing with other parents and need more experienced source of advice. The book is on Amazon. Read it first. You may find your own answer.