Sounds like your sweet girl is having a rough time these days.
There are some great "quality of life" scales available that might help you with this difficult decision. Input from a vet you trust can also be invaluable. I so sympathize with you, it's never easy to know exactly when the right time is and trying to find that balance between not wanting our pet to suffer, but also not wanting to end their life too soon if there's a chance for recovery, or for more good days than bad. Such a heavy burden to bear, but also such a huge gift we can give. My heart goes out to you.
http://www.hilstvet.com/quality-of-life-scale-pets/
http://www.lapoflove.com/Quality-of-...uality-of-Life
http://www.homepeteuthanasia.com/end...uality-of-life