You did a lot.
in fact!
You should not feel like you are to blame.! It is your mind getting to you - 'cause you did bond with her. Tell your mind to take hike -
you're a good person & from the number of pets you do care for - I'd say a bit of a pet push-over.
Not all of us are blessed with perfect health or perfect situations. All young things are fragile - even a plant. Yes she deserved better from the breeder all the way to the store owner. You could not take her - that was clear.
You did your best
& I thank you for being there to stand & bear witness.
Thanks for telling us.