Here is my training story. Some agree, some think I'm stupid so take it with a grain of salt. hope it helps as I have been in a similar situation
My youngest Cav Sprinkles was extremely reactive around other dogs. It was my own fault since I didn't properly socialize her until 6 months of age. She was my "baby" and so I spoiled her rotten, causing multiple problems when it came to other dogs. By the time I took her to meet some friends, she was TERRIFIED of other dogs. She would react by barking and whining excessively and sometimes even nipping at other dogs. Needless to say, I was horrified!
I researched many different training facilities asap. I found the most expensive one that I will not name out of respect. It was a training facility that focused only on basic training, agility, and obedience. Our first class day came and I took my Sprink. I was prepared for the worst... and lets just say I am glad I was. They stuck us in a corner of a quiet room so that we were one on one with another trainer. She was not exposed to other distractions. She was not exposed to other dogs. We wasted 4 weeks of "training" like this. As the weeks went on she only got worse. While walking into the training facility with other people and their dogs Sprinkles was horrible. My little 6 month old 7 pound Cavalier was actually scaring people. I was confused as to why we had worked so hard with this "puppy socialization and training class" and she hadn't yet been around another dog! Their response was that she'd learn with time and age how to behave around other dogs...
We quit after 4 weeks since it obviously was not helping. One of my friends actually recommended we try the training classes at Petsmart. I refused at first. I honestly thought it was kind of ghetto. I didn't like the idea of training my dog in the middle of a store with people watching.
After a few weeks of thought I decided to give it a shot. Our trainer explained the first puppy class that there they would learn not only to get used to other dogs but also distractions of everyday life. I mean afterall, what is more distracting to a dog than petsmart noise? I wont lie. Sprinkles was HORRIBLE those first few classes. She spent the entire time barking excessively. Whenever another dog would get near her, she'd lunge at them. Our trainer knew EXACTLY how to handle this. By the 4th class or so I started to see improvement. Our trainer spent individual time with Sprinkles as well as with the whole group. We would go to petsmart to practice socialization EVERYDAY. And everyday our trainer would be there to help... she didn't HAVE to since it wasn't our class time. But she was ALWAYS there giving me tips and pointers on how to SAFELY let Sprinkles get used to other dogs.
I am proud to say that 6 months later Sprinkles is now in her 3rd week of advanced class at Petsmart and is training for her CGC. She now absolutely loves other dogs and we are looking into doing agility and therapy. She will occasionally have a reactive moment while leashed (Our trainer helped us to figure out right away that Sprinkles had reactive leash aggression) but I am now prepared to handle it and move on.
I know it's hard to socialize your dog with other dogs. My dog WAS that mean dog hahaha. But as long as you find a good trainer who you are comfortable with, one that gets to know you and you're dog, it is completely worth it. I have since started all of my Cavs in training there. I had NO idea how "untrained" they were until we met our amazing trainer.
I hope everything works out for you, and don't give up!!