Try the same park consistently instead of a new park each time. She should get to know the area and some of the frequent visitors over time and come out of her shell. Don't coddle her either. When she chases another dog away and then turns around and comes to you just let it go. If the other dog reacted well by leaving her alone it will help with her confidence that she can tell other dogs when she want to be let alone and she's not in any danger. She may soon realize the other dogs aren't interested in her when she does that and then she will have to go initiate a greeting if she's curious about them.