I'm so sorry you're going through this. Our cat died almost a year ago and I was so grateful for the support and help I got on MN.
Like you, I was in two minds. He seemed fine, but wasn't eating at all and losing a lot of weight. We took him to the vet, got lots of tests, and was found to have lots of tumours. Decided to take him home for one last night before being PTS.
I was in bits. He was jumping about, purring, seemed in no pain, so I was wondering whether I had rushed into it. I got such kind words here and people who knew more than I ever could told me how it could all change so quickly, that he could be in terrible way within 24 hours even.
So I stuck to the decision and I really do feel it was the best one. He had his last morning lying in the sun, and it was all very peaceful.
I think we're incredibly privileged to have animals in our lives, but we have to make some tough choices. I would have hated for my lovely cat to just disappear off to die, and I hope we did the best for him.
I think all I can say is that you know you love him, you know you'll be heartbroken, and you just have to accept it will hurt you - but his needs come first and it just sound like it's time.