I am so sorry to hear about your cat e beswick. I can only tell you what we decided to do when we were told the same about our cat a few years back.
We'd had our beloved Smudge for 13 years, when she unfortunately got cancer of the knee. The Vet adviced that they could either amputate her leg, or put her to sleep.
We took her back home for a few days & after lots of agonising, decided it was best to put her out of her misery & have her put to sleep. We all cried buckets, but knew it was the best thing for her.
My husband & youngest daughter stayed with Smudge whilst the Vet gave her the injection - our other daughter & I waited outside. We then brought her home & buried her in our back garden.
Thinking of you. -x-