If questioning the decision to spay helps you to make a different decision on that in the future, then that's ok, but then let it go. You do not know that the early spay caused this issue. Maybe the cells were there, and the early spay knocked her out of balance and let them get the upper hand. Maybe the onset was younger that it would have been. Maybe... There are a lot of maybes and that way lies insanity, really.
We do not know that the cancer is even one that is linked to early spay/neuter. I haven't heard of leukemia/lymophoma in GSDs. But maybe they just call it different stuff.
The thing is, we can, all of us, beat ourselves up about the choices we made for our critters, and play the blame, built, shame game. We can't always know what is the right thing to do.
You have to hang onto the fact that you gave her a good life, and did everything possible to help her, fix her, not let her suffer.
I am very sorry for your loss. Be kind to yourself. And let yourself love more dogs when you are ready.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.