Oscar, my little boy

Fifteen years ago I received a phonecall to say that a cat had been thrown out of a car in Peterborough. When we found her she was giving birth and luckily all five kittens survived. Mum and her babies were handed over the to the Cats Protection League and I would phone on an almost daily basis to see how they were doing.
Eight weeks later three of the kittens had been rehomed but nobody was interested in a tiny scrap of a ginger boy or his black and white sister.
So Oscar and Molly came home to us. We had three fantastic years with them (they fitted in well with our zoo!) when Molly was hit by a car. It was devastating but Oscar was inconsolable. He would sit by the door and make crying noises for hours.
Years past and we received our posting to Gibraltar. By this time Oscar was twelve and Rupert (another rescue we had from him being four weeks old) was fifteen. We knew we couldn't take them due to the heat and their age but I couldn't leave them either.
My wonderful Dad and his wife Tracey then said they would have them. He lives in a cottage with a huge garden and they would be safe. So the tough decision was made.
So now we are back from Gibraltar but there was no way Dad and Tracey were going to hand them back. They had fallen completely in love with them...which is understandable.
Last night Dad rang to say he had bad news. Oscar has had to be put to sleep due to kidney failure.
I am gutted. I feel so guilty for not taking them to Gib with us and for not being there at the end for my little boy. I know that he had a lovely life...but we are all going to miss him so much.