mate, it's heart breaking, but you've got to think of your dog. would you rather her be at rest and not in any pain, or would you rather her up to the eyes in meds, but still suffering because she can't get up and run about?
My missus had a collie and he was about 16 years old when he was put down. He had terrible arthiritis in his hind legs and putting him to sleep was absolutely the right thing to do, you could tell the poor dog was ashamed at being so feeble.
It'll hurt you parting with her much more than you'd ever imagine, you'll have to be brave and do what's best. It's amazing how dogs can affect you, but you'll look back at her with nothing but affection and once the pain has subsided, you'll be able to look back and smile.
YNWA