Joker is very lucky to have someone like you. Not only did you give him a second chance at life (which sadly, not many people are willing to do), but it sounds like you couldn't love him any more than if you had him all 15 years.

I think you just know when it's time, when the bad days outnumber the good. It's the hardest thing in the world to do, but sometimes we need to look beyond our own pain and end that of our beloved pets. I'm sorry you may be facing this.