I am so sorry. I too know how hard it is. My boy had cancer too and it seemed to have come out of the blue as he had just been to the vet a few months before.
Letting them go is the final gift we give them but no matter how right it is it hurts beyind belief.
Godspeed Sadie girl, I hope my Cyrus is showing you the ropes.
Mom to the late Cyrus the Fluffweiler CGC forever in my heart and James the Alpha cat