My Border Collie was diagnosed with hemangiosarcoma the day my late wife died of cancer.......he seemed to be doing so well, and I'd been in contact online with other dog owners in the same situation who made helpful suggestions.
About 8 months later he I and the cat were in the 5th wheel heading for AZ......we went for a walk..he jumped in the truck.....by the time we arrived he'd lost the use of his back legs.
Had a comprehensive test done at the vet's....they said it was all through him.....so we gave him the final shot.
I went back, sat in the truck, and cried like a baby.
That was around Thanksgiving 2002...........it's eased, but I haven't totally gotten over it yet.
Hang in, all the best, but it's not going to be an easy ride I'm afraid.