My shih tzu is afraid, trembling and uneasy when we go to the pet shop for her regular grooming, what to do?

Provided she didn't suffer any trauma on previous visits, I'd suggest going to the grooming parlour isn't much better, for some dogs, than going to the vet. In other words, her reaction may be understandable. Might it be worth considering finding another groomer (perhaps her breeder would do her for you - many do?) or staying around while she's being done, rather than leaving her and walking out?