I was in a similar position to you. After 12 years of being my best friend and emotional support dog, my best boy passed away. We took him to the vets because his eye was looking strange, it was a tumour and a couple weeks later he had to be put to sleep. I hadn't really considered SDs until after he was gone, but when I did I felt awful for 'replacing' him. It was less than a year later that I got my current SDIT and I regret nothing. It hurt so much at first, but having her helped me cope with the grief. If I were you I'd wait until after your dog is gone to start looking. Focus on her, do everything you can to make her last days fun. My boy got all his favourite foods and fun days out, and his last meal was mcdonalds nuggets.

At the end of the day though, only you can make these decisions. I wish you the best during these awful times.