I am so sorry about your little pal. I can only tell you the lesson that my beautiful setter taught me:
I did everything for him from top to bottom, I poured funds him into my beloved guy. I watched him in the end suffer, and I decided to put him down.
What he taught me, everything I did for him was for me, not for him, and he said it is time, I am having a hard time. I made myself feel good not him. He was only 7 years old.
I was offered chemo for him to live a few more months, the day I put my boy down, he was actually rotting and could hardly walk. I told the vet no way, he deserved better then that. He deserved to be without pain, and be at peace. His journey is etched in my mind forever.
This was my experience, not yours. You have to do your soul searching. No doubt its a hard situation to figure out what is right for your forever pal. I wish you well, and in your heart you will find the right thing to do eventually.
Prayers coming for you and your family.