Not the best at puppy information.
Breath deep and hang in there. Do get into the class when it starts up again. How about working with Butters in the back yard or in the house on the leash. Tons of treats. Put one right in front of her nose and move slowly forward. Praise, praise and praise some more. Little ones have the attention span of a gnat so make sessions short, 10-15 minutes.
It's a big world out there, so many things never encountered. Start, again, in the back yard or even in the house, introducing new stuff. This can include new surfaces to walk on, a blanket, newspaper, crinkley plastic. Introduce new sounds, new objects. Archie, at 5 years old, was not socialized much with the outside world and some things still bother him. With slow, gentle introduction, he is doing great. He actually met his first motorcycle a few weeks ago and did excellent. I don't agree with your vet in that Butters can't be properly socialized. You need to be aware of something new and approach it with confidence; that confidence goes right down the leash. Don't push it or rush a situation.
Hope this helps some.