Socializing Scared Rescue Pup

Hi! 3 weeks ago, we adopted our sweet pup Daisy (5 month/f) from a rescue shelter. She is a very timid/fearful dog, but we've given her lots of love and she's really warmed up to us and is very comfortable in our home/yard/neighborhood.

We decided to try a puppy class at a Petco today and it went terribly. Our poor girl was immediately terrified when we walked in; she peed and pooped then laid down, shaking. We cleaned it up and eventually she even walked about 15 feet into the store, but she was so scared the entire time and the trainer said to get her out of the store and she would give me a call later, where she told me she was removing her from the class as this wasn't the place for her. I respect and appreciate that; I think we all knew we couldn't keep her somewhere she was so deeply afraid.

I want to keep trying to socialize her, but I'm not sure where to start. She wasn't afraid of the other dogs-- the reason she got up and moved in the shop was to go see another pup. But I'm afraid of doing more harm than good if I keep trying to bring her places when she's terrified.

Does anyone have any experience with rescue pups where they can offer some advice? They didn't really have a history when we adopted her, just that she's been bounced from shelter to shelter her whole life so I don't really know the level of trauma we are trying to heal.

I appreciate any advice.