Hi, welcome to the forum,
What a complicated situation!
Why did the breeder take the dog back to train him for separation anxiety? (Taking him away to train for SA isn’t going to help his SA… If anything, it’ll make it worse).
Did the breeder collect him, or did you drop him off?
Has the breeder given any reason why she isn’t willing to return the dog?
If you’re in England or Wales, you could try Trevor Cooper at https://doglaw.co.uk/about-us/. Or maybe Citizen’s Advice.
Did the police give you a crime number?