We had a MT's exbreeder previously and she was the sweetest thing. I would say it took 6 months to a year for her to find her feet. We had two very confident crazy cocker boys as well, and she learnt more from them than she ever did from us. Firstly, a lot has happened to this wee dog in the last few months. From never being out of the breeding facility, she has now been in a rescue kennel, possibly a foster home and now her forever home. So many new things, smells, sounds, textures. Do you have a 'safe spot' for her? We had a covered crate, and that was off limits if she was in there. When people come to visit, can you have her in another room, even with a baby gate to close it off? She needs to feel safe. New people will scare her and she will lunge through fear. I would ask visitors to ignore her, let her be, just to get used to various people. It's good to have new experiences, but they can become overloaded. Time and space is what worked for us. After a while Lara would greet people, but always a distance away and very cautious. The boys were overly social! She used to stand back and watch in amazement
You are not doing anything wrong, these wee dogs just need a bit longer and a little more understanding than a well socialised and cared for dog. One thing I discovered, and I am till annoyed that I did it, is that squeaky toys reminded her of pups. There were 2 that had a fairly sharp pitch, and she mothered them relentlessly. If I had to do it again, I would not have squeaky toys. Also she never got used to walking through a door, she would always bolt from one room to the next. Some things will just be the way the are, even long term. Just do what you are doing, and manage interaction with viitors carefully. I think that is amazing progress for 3 weeks.