First let me say how sorry I am for your loss. Nothing feels as bad as loosing a loved  one and our little furbutts are every bit as important and loved as any human in our lives.   
Next I want to say God bless you for rescuing her from a horrid life. I know she was able to have at least some joy in her life before she left you.   
Finally I want to tell you that I have loved and lost many dogs. Each time I second guessed myself. I chastised myself for not doing something sooner or seeing the signs or not recognizing illness. I even once accused myself of laziness because I was asleep when my baby died.  I'm telling you this to let you know that it is normal to wonder what you could have done differently or better. It sounds to me like you did everything you could. We are supposed to trust our vets and believe that they know what to do with our little ones.     
Don't let this turn you away from loving another yorkie in the future. You are exactly the type of people that those little lost souls need.   
God bless you 
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