oh boy , lots of problems , serious problems .
You need to go right back to the beginning . You brought home "She was very shy as a new puppy " .
could you tell more about the pup , her dam, the handling and exposure she had while at the breeders. Chances are she needed more good socialization before you brought her home.
If the dam was anxious while in whelp , the pups would have been affected by this . After they were born if the dam was anxious the pups would have copied behaviour , learned flight avoidance . If the dam was an anxious sort , chances are that there is a genetic base in addition to environment .
That "quite energetic and nippy." could well be anxiety .
this is so important "I have socialized her (and continue to) as best I could, with people and dogs. Dogs, great, people, huge fail."
how exactly did you "socialize" her? Dogs , no big matter. People huge fail will define her future.
A dog can live without further exposure or contact with other dogs .
A dog can NOT live without being in contact with people , and now you see she is anxious with you to the point of growling .
so you have to go back to square one and deal with the underlying problem , which is a lack of confidence and trust in people. Slow and cumulative positive steps, at the dogs own pace .