New beginnings and letting go of your dog?

Nala is a Nova Scotia Duck Tolling Retriever mix Brittany Spaniel. At 12 years of age she is slowing down. She has severe hip dysplasia and a severe heart murmur coupled with an underlining heart condition. She runs after the other pups when they bark and run out the dog door, ocassionally plays with the other pups, and is still eating a bit.

However I know she is getting older, as of now she is being treated for the pain of her heart and hips. But saying good bye or letting go will be hard. I feel soon she will need to put down if she doesnt die first. Ive heard I will know when its her time, but I feel that within a few months her time will be limited… should I go with my thoughts, or wait it out? As for new beginnings Ive also heard that getting a puppy non resemblent to your previous helps in the passing. Needing time to grieve but not for too long. When should I get the new pup?

It seems I have failed to mention. I own four dogs. Two Border Collies, an ESS (English Springer Spaniel), and Nala. I feel they will suffer as much as I will. It’s hard deciding when to get the new pup, whilst myself and the other pups are mourning, or before hand. There are four dogs, but two different packs. Kei and Harpoon (the two Borders), and Nala and Jasmine (Mutt and ESS). Jasmine is dog aggressive, and luckily was introduced to Nala at a young age, I feel this will be a big blow to her, Nala has been with her through it all (Jasmine is 7). Its rather hard to say if I should be choosing the pup I feel is right or I feel Jasmine will adapt to better.