Very sorry to learn that another good feline soul is getting close to passing away.
You will hurt, but once your cat friend is gone, he won't hurt any more.
One of the best friends I ever had was a smart, bold Burmese cat, who made it obvious, every day, that he thought I was the greatest thing that ever happened to the world (yes, even when he wasn't hungry). We lost him at the too-young age of 8, to some mysterious blood disorder the vets could not identify. They kept "observing" him, administering one test after another. I won't go so far as to say they were using my cat friend to experiment on at my expense, yet when I went to visit him in the pet hospital, he was so ill and miserable he didn't even know who I was, and had lost control of his bladder. He was being kept on a water IV because he wouldn't eat or drink. That's when I said, "Thank St. Francis that my friend is not a human being, who must be kept alive at any cost--I have the power to let him go." And I did, though I have missed him from that day (1995) to this.