Kelly I am so sorry to hear about Muffin. Please don't blame yourself, it wasn't your fault. But I would consider taking action against your old vet. I can't believe he never noticed it!! I would be furious with him.
The important thing is that Muffin is getting treated all thanks to you. You are such a wonderful mommy to her.
I will pray that her surgery goes smoothly and that she will have a speedy recovery.