I am so sorry to hear of your sad news but I have to agree with Bathsheba - if she's in such pain, please let her go sooner rather than later. She won't feel a thing, honest, the vet will treat her with dignity and respect and you can hold your head up high, knowing she had a good life and a peaceful passing. I could not bring myself to attend to my two previous pets passing on either, in fact I am sure that few loving owners can do this - no shame there either.
You will find that your other Staffie will be wandering around the house for a few weeks, wondering where on earth the Collie has got to, s/he will surely miss her too.
Unfortunately death is part of living - we all have to say goodbye to those we love, just be grateful that you gave your dog a loving home and remember all her daft antics over the years. xx J