Oh Kate hunny, i feel so bad for you. It's a terrible decision to make, it really is.
Could you call the vet for a chat? Tell them what you've told us, that way, although it's ultmately your decision, the vet may help you to realise how kind you are being in making the right decision choice for your precious cat.
i only say that from my own experience with our rabbit who was running around the garden like there was nothing wrong, but inside there was and to undergo an operation would have been distressing for him and he may not have even survived it. It was an awful time, i cried buckets as i spoke with the vet, she assured me it really was the right thing. It helped knowing that she thought that too.
big, big hugs for you