Stop taking her. She's telling you its scary and she doesn't want to be there. Why force her? Most Gsds aren't dog park dogs. They prefer their pack and really don't care about anyone else. Listen to the poor old girl. Dog parks are notoriously pia places, that are more for the benefit of the humans (most of the time) than the dogs. Are you really going to force this on an 8 year old rescue dog? Enjoy her with your family, go for walks, hikes, car rides.... Play with her, interact with her. That's what she wants, not to be forced into a place she's backed out of her collar at and ran for the door at.