Our rescue pup was like this when we first got him... Cowered at the vets, and when I introduced him to our trainer. Barked at everyone on walks.
I introduced him to new things slowly, and repeatedly. His first walk at the park was stressful for him. But I took him again and again, and now he's great. Petsmart was the same thing... We just praised him a ton, did not coddle him or pet him when he was afraid, worked very hard at the "leave it" and "look" commands and over a few months he was greatly improved. He can go to any park, any store, the vets, etc. with no issues, and he has lots of fun. I won't let people or dogs approach him if he's acting nervous, and I keep him in a stay when people come over until he relaxes. Then he can greet the guest and get some treats. I try to make sure he knows it's my job to handle things, not his.
I don't believe his is naturally a very fearful dog, but his confidence needed a lot of help - occasionally a certain person or dog will make him nervous, but overall he's gotten a lot better. Hope this helps!
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