To the OP I can see that you are concerned about your child's issues and they may very well be cause for concern. Fear of change is a real phenomenon. When a child or an adult for that matter measures their existence in terms of what stuff is around them then it is time to involve professionals to get to the real reason. It is partly a form of severe separation anxiety, partly a self-esteem issues ( I am the sum of my possesions), as well as under certain syndromes that involve mental health issues there are true instances where the child/adult can not handle change of any kind. They may actually need professional counselling to learn to let go and not be so rigid. If it was just her toys yes I understand that kids get possessive. But to fret over a napken - that she has only had for less than an hour at a restaurant or the family car indicates that it is time to speak to your doctor and express your concerns and ask for someone to advise you.

I get that you have to carry through and do things like trade the car and she will just have to get over it. But realize that her anxiety is real not pretend and needs to be considered and she needs to be given coping skills to handle it.