Eis, Just be straight with her.
I explained to mine years ago, how much my sporting life means to me. I explained that gaps in my dog ownership meant seasons of dissatisfaction. Years I would never get back.
Dogs don't live as long as people, and there would always be periods of overlap, when one dog was old and retired, and one was young and rambunctious. So far, barring my failed litter 2 years ago, it's been a good run. I'm sans Lab right now, and will fix that as soon as I can, for next season. With her understanding.
Dogs fill a place in our lives, and she knows I can't be happy without them.