Hi D.- your daughter sounds a bit like mine, and our circumstances sound similar in that we also have another daughter who is very easy-going. In my situation, however, the two girls are twins! They just turned 9. I know it's so stressful and tough on you, never knowing what your daughter may do,or if she'll do "the right thing". My daughter has sensory issues (dysregulation, overload) and is often anxious/stressed. We have had her evaluated by a neuropsychologist, sees a counselor regularly, and has received OT services for sensory regulation and motor planning. She also has a lazy eye, which is nearly corrected. The two things that I think have been the most valuable for us have been realizing that the more stressed we get ( or me especially, since I'm the one who's with her the most), the more stressed she gets. If I can try to relax, and "let things be" about situations, it makes it a lot better. The more I push, or nag, or remind, the worse it gets. The second thing that has helped tremendously is her maturation, sorry to say. She has been able to better control herself, express herself the older she's gotten. It's also helped a lot to have had her in therapy, to help her learn to use her words, and find ways to relieve her stress. Also, she received OT services at OTA in Watertown for one summer. I learned a lot from them. It was very expensive, not covered by insurance, so we couldn't continue, but the techniques I learned, and she learned, to help her were very helpful. Now, I can anticipate how she'll react to the stimulation of a grocery store, and prepare her for it in advance, and have her do things that will help her, such as chew gum, push the shopping cart, get specific items off the shelf, etc.
Lastly, both my daughters go to a small private school, where her issues can be more easily addressed than at a large public school. The teachers and I work very closely together to help her help herself.
It's been very stressful on our family life, too. The counselor has helped us all work together to deal with issues that are problems for us. I hope this helps, and I hope you can get the help you need. Don't try to handle it alone, it's too big a problem. Be consistent, give her a structure/routine and lots of affirmations that you know how hard it is for her.
Best of luck.