Good on you for taking in a dog having a hard time, I know how hard it is when you have to put them down, it's an absolutely horrible thing to have to do, but sometimes, it's for the best
As long as she's had a good ol' life while she has been with you then that is all that matters
We have always had rescue dogs, we have two border collies at the moment and one of them was kept in a garden for two years, wind and rain, living on scraps before we took her in..
I'll admit it like others, I get really attached to dogs.. The last dog of mine that had to be put down was my best friend, and I cried and cried and cried, but, it was the right thing to do
Hope you consider saving another poor dog mate, chin up