I'm so sorry for your loss...you never expect these sort of things with younger animals and it makes it very difficult.
I had to put down my 6-year old golden retriever a few years ago, after a particularly nasty and expensive battle with cancer. We dug-in and fought through two surgeries and ultimately it still ended up spreading through her internal organs. At that point there was nothing left to do, and I felt terrible that she suffered all the medical procedures for nothing...even though it was good intentions and hopeful thoughts. Personally, I feel you made the right choice. My sincere condolences