Dear Sarah, I am so sorry for your loss! Your post took me back 15 years when my dog had the same diagnose and I had to make this decision. It was the hardest thing I have ever done. I told the vet to operate and they did, my dog died on the operating table.
I was concerned more with my own conscious being clear then the suffering I put my dog through. I would never do that again.
You did the right thing.