Nina, I am so sorry to read this. Your experience mirrors the one we had with our first cocker,Millie. We had to have her PTS last year - very similar behaviour to Maisy.
I have to say, when I read your post about how Maisy was behaving, it sounded just like what Millie had been like at that age. Her biting was not a puppy bite either, it was a proper attack.
You have taken the most difficult decision any pet owner ever has to take. I still blame myself and miss my Millie every single day. I have not yet been able to look at a photo of her since she went. Dogs who have the history that Maisy had should not be rehomed. For me, rehoming Millie would have left her open to being punished for aggression that she had absolutely no control what so ever of.
If you're interested, and when you are feeling stronger, you can read about Millie's story in the looking for a puppy section of the forum.
One last thing. Please don't let your experience with Maisy put you off cockers for ever. They are fabulous little dogs. I learned so much from Millie. I now have 2 cockers who are true to breed in terms of temperament. They came from a good breeder and were well socialised.
Nina, I know in my heart that Millie just could not be saved, and I hope that, in time, you will appreciate that this was the case with Maisy too.
You, and your family, are in my thoughts.
Please PM me any time.
Take care.
