I feel your pain. Rosie sounds exactly like our dog, bc x kelpie, brought home from a shelter at 9 months old. With the help of a behaviourist, we worked hard at behaviour modification, gave heaps of enrichment, exercise, training, but he was not getting better. Working dog, dog needs a job, herding, agility, flyball people said. Yeah right, he hated the car (yes despite three months of behaviour mod we couldn't get out of the drive way without a melt down) and was anxious around other dogs anyway, and incapable of retaining training. Restless, slept maybe 6 hours a day, total. He couldnt even sleep, clearly he was not able to retain foundation learning or bonding, his anxiety over rode all of that.
We were referred to a vet behaviourist by the first behaviourist but resisted because we didn't want to go down the medication route because we didn't believe in medicating dogs. In the end we were exhausted, had moments of resenting the dog for not responding to all our hard work and making life so bloody difficult, seriously wanted to rehome, and pts was on the table. We went to the vet behaviourist and decided to try medication. Within three months all the behaviour modification clicked into place. He started to relax. He's been on medication a year now and we are going to try to wean him off in the next 6 months, he may or may not need to stay on the medication for life. He's such a happy, energetic, cheeky sweet dog, not at all the drugged dog I feared he would become. Our biggest regret? Treating medication as a last resort rather than an "in combination with behavioural mod". He suffered longer than he needed to.
So yes, my advice to the op, given our experience and given that despite your best efforts things arent improving, is to see a recommended, qualified, and anxiety experienced behaviourist or a vet behaviourist for an assessment, and trust them. Best of luck, I hope things improve soon.