I haven't back through your older posts, so I'm just going to go from what you've written here.
You mention your daughter is introverted and sensitive.
Sometimes introverted quiet kids get really stressed out if they do too much in a day. I'm introverted and quiet. If I am hanging out with my really extraverted (go go go) friends, I get wiped.
My good friend has a husband who is hyper. When he's home, their child gets stressed, anxious, shows a totally different side to herself. It's like she overloads on his energy. Everything then becomes a huge deal and she's very emotional.
So - just wondering, what's the rest of your family like? You mention her older sister fights with her - can you stop this? Friends over ... maybe it's too much for your daughter?
This particular friend I have has to limit playdates to an hour or two max. One child.
Her child did do therapy, but ultimately they just weren't listening to her. She was stressed.
It's hard to say (I think you could benefit from the therapist and so could your daughter.. even just a session or two) - but I look at environments when kids get stressed or anxious. Another friend of ours had a really anxious child yet he was in competitive sports - 3 days a week. They took him out of it (down to rec) and he was able to go off his meds.
Just a thought. Anyhow, I'm sure it's nothing you have 'done' - although I get why it's easy to think that. We parents tend to blame ourselves!
Good luck :)