Don't second geuss yourself. I put a dog down a couple years ago. It was probably one of the hardest decisions I've had to make. She had a tumor the size of a football hanging from her tummy. The vet said she could remove it, but then in a few months, I'd be back in the same boat.
It was the right decision, but there is not a day that go's by that I don't think... "What if..."
You made the right decision. You used your heart, you used your mind. Hearing statistics is just going to make your "what ifs worse". Just know you made the right decision for that moment.