We got Lila about 3 months ago from a shelter - she is a 3 y.o.
lab/terrier mix. My husband has usually been home during the day so
there has been as yet no need to hire a dog walker or take her to day
care on a regular basis. But yesterday I took her a day care group as
my husband is visiting relatives on the west coast and I didn't want
to leave Lila alone all day.

I felt confident about the reputation of the day care center and about
the appearance/owner when I got there, and was surprised to find a
completely nervous wreck of a dog when I came to pick her up. I had to
leave her in a kennel for two days over the holidays (relatives
couldn't have her in the house) and ditto - she was beside herself
when I came to pick her up, and even continued wimpering on the ride
home. I know she doesn't realize (and there's no way to make her
understand) that these breaks aren't permanent, and that we're coming
back to get her eventually. She doesn't get all neurotic when we leave
her alone at home for a few hours - this seems to happen only when
she's in an environment where there are other dogs.

I would have liked to take her to day care maybe once every one or two
weeks, just so that she gets better socialized and gets used to other
dogs, but I'm wondering now if that's a bad idea, given how panicked
she was yesterday. I know she lived with another dog in her previous
home, and when we've encountered other dogs on our walks, she is bold
enough to go over and give their butts a good, long sniff. In
training, the trainer told me not to pet her when she's nervous as it
reinforces her nervousness, but to speak to her in soothing tones
without touching her. I'd like her to become more secure in these
environments, but don't want to push her either if she's not ready for
play time with other dogs. How can I help her calm down?