I am so very sorry for you Jools.
I have been in a very similar situation as you with another breed, and the guilt is sometimes unbearable. Trust me in time, you will begin to see much more clearly that this was your only option, for Millie, you and your family.
You are certainly not to blame in anyway for Millie's problems, you did your upmost to help her but some things are just beyond our control, you were very brave and you did the very best that you could do for Millie.
I think you are especially brave and I thank you for posting about your experiences. I know how nieve I was when looking for my first puppy and I am sure that this will help inform people and help them to avoid the heartbreak that you have endured.
Run Free Miss Millie