Hey there! I am so sorry to be reading this. I completely understand what you have been going through. My cat went through a similar experience. After breaking her rear right leg, she was put in a splint for what was supposed to be two months and ended up being three because it wasn't healing properly and needed additional surgery. After all of that, she was finally back to normal for only a month until her leg started bothering her and it ended up having to be amputated two weeks ago. 

Athena too has been sleeping more and only just started really playing again. I agree with others in that their pet developed a "new normal". This makes me sad, but I think it's for the best because our pets are no longer in pain. They are very resilient and adapt well. 

It sounds like Dexter has some great cat parents. I hope he continues to do well.