It really does sound like euthanasia is the kindest thing you can do for her then - it is the last act of love and kindness we can give them, although never an easy decision, and sadly it never gets easier. Good luck
I believe that a good vet can make a world of difference when deciding if euthenasia is the best choice. I trust my vet completely, but several times in the past when I've asked if I should make that final decision he told me the decision was up to me. Finally, with my sweet Sophie (who also had cancer) I asked him, "If Sophie was yours, what would you do?" He looked me in the eye and told me, "I'd end her suffering; she's in pain and she's not going to get better."I don't know if this will be of any help, but this was my experience last year. I put my cat, who also had cancer, to sleep when the vet told me he was actively dying. There was such a pronounced difference in the way he looked, in just two days, that I "knew" but I think the vet saying that made the difference for me.