wow... I know this must be hard.
I love shih tzu also. I have one, and he is the sweetest dog to his family there is. He doesn't like strangers at all.
Before I got Remy I wanted another shih tzu and would go onto rescue sites to look... many are listed as not getting along with other dogs and should not be places in homes with children. I think the females maybe more difficult than the males.
I will say.. Bailey- the one I talked about above -- has improved a lot. It took time. I also have a lasha who had a very .. independent almost aggressive personality -- we had to get really tough with him. Think, it is like children, they want to be bossed, they want limits... crates are not as bad as we view them. I don't crate mine now.. but have when needed in the past.
The pup is not adjusting well to you home, could you get a trainer or maybe advise from your vet on how to help her? Maybe some calming drops.. or thunder shirt to calm her?
I wish you the best.. do what you can. If you give her to rescue be sure you have done what you can and can give her with a clear heart. I know there is someone who would love her. .. if I lived closer I would give it a try.
(I need another dog like a hole in the head right now
)