My dog used to be absolutely terrified of the groomer. He has a lot of hair and the noise of the clippers just drove him crazy. He was never aggressive, but instead just tried to curl up in a ball and wear himself out trying to "protect" himself to the point of exhaustion. It was so sad to watch.
After four years of watching my dog howl in terror every time he was groomed, I reached my wits end and decided something had to be done. I started researching why some dogs are so scared of grooming and what can be done about it. What I found is that our groomer, while not being malicious whatsoever, didn't "listen" to the cues from my dog when he needed a break. He just plowed through to get the job done. Apparently that was a bad miscalculation on his part.
I thought I had permanently damaged my dog to the point of never being able to get through a grooming without having him take a ton of medication, but after some casual reading I realized that the age old saying "you can't teach an old dog new tricks" isn't a very accurate one. In fact, middle-aged dogs can often be easier to train because they aren't as distracted as puppies.
Thankfully I found a new groomer who "started from scratch" with my dog. The first couple visits to our house (it's a mobile grooming company) involved "play dates" in the van. My dog literally just went in the van for 30 minutes or so, sniffed around, got to know the groomers smell, and got to eat a lot of treats. After a couple "play dates" he was ready for grooming and I'm happy to say that he loves it now.