Something that comes to mind upon reading your post is that perhaps Sadie didn't have to gain independence/experience new things because it appears she never left her litter until 3 months when you got her. You commented that the breeder had both parents and all the brothers and sisters on site when you purchased Sadie. And you commented that you had her for a month and she's now 4 months old. Its odd why the breeder would still have the entire litter of puppies at 3 months of age.....normally they would have gone to their new homes by 2 months. Poor Sadie never HAD to gain independence and do things without the support of the litter behind her.....and then when you did take her she well could have been going through a fear period....I think one of the fear periods is around the 10-12 week mark.....
I'm glad that you have a supportive trainer and are working through issues with her!! Keep up the good work and I'll keep my fingers crossed for you that everything works out well.....keep us updated on her progress
Heather