Thankyou so much for all these replies. I've lost other cats before and I know the sadness and pain it gave me, this is the first time I've had to make the 'pts' decision and also the first time I've had surviving cats at home. But your posts have at least made me feel normal with these guilt feelings. Sometimes when I manage to forget it for a moment, I feel guilty for feeling happy too. Maybe she'd been ill for longer than I realised, perhaps I could have helped her more.
She laid her head in my hand as she fell asleep forever, and I keep seeing and feeling that moment, and I'm angry I can't bring her back.
There are people and animals that have much worse lives than us, but I really didn't expect Ribs to be the first to go, and so soon.