A.C.
This is going to be long, because I too become deeply attached to my animal companions.
Animals don't understand pain. They don't understand bodily failure. All they know is that it hurts. This is one reason why many vets don't recommend chemo for animals. They don't know you're trying to make them better. They only know it hurts.
We put down my family dog the year I graduated from high school. She was visibly in pain, her time was coming. To keep her alive would have been a selfish act for US, not for her. She wasn't responding to our voices or touch as we drove to the vet, but I still remember that, right before we put her down, she lifted her head and looked around the room and made eye-contact with each and every one of us, even though she had to turn around to see one of us. I have always believed that she knew the end had arrived, and was saying her good-byes. She wanted to make sure each of us knew that she had loved us.
I am crying as I write this now, because the memories are bittersweet, but I think you need to think about that meow in a different way. Your cat was in pain. He knew that the end was coming. Had you chosen to delay, you wouldn't have saved him. He was old and very sick. Maybe you would have extended his life, but that would have meant you just would have extended his suffering. And in his last moments, he said good-bye to you, to let you know that HE KNEW YOU WERE THERE WITH HIM, and he would miss you. I think that is why he meowed.
Losing a pet is losing a family member. Allow yourself to grieve. That's okay. But don't feel guilty. You did what was best for your cat, even though it brought you pain. That is what a good parent does.