I think you should continue to love her and treat her in the wonderful way you have all these years. I had to make the choice with my bulldog who had brain cancer. It was one of the hardest things I have ever had to do, I miss him every day, and its been 2 years. Sometimes the greatest act of love is to let them go, its also the hardest. Enjoy the better days and get through the worse. When the time comes let her go with all the love in your heart and know that she was truely blessed to have travelled her life with such a loving and unselfish partner. Hugs and strength to you through this very tough time.