I know exactly what you are going through. I just made one of the most difficult decisions in my life, and had a dear, dear kitty of mine, PTS. I couldn't do it anymore. I know she went peacefully - purred her way to the Rainbow Bridge. She is no longer suffering. She was ill, was tested, treated and then some, and she wasn't improving. I made a difficult call, but now, know it was the right one. It took me years. Yes, years. But I knew when it was time to let her go.

Good luck with your decision. Only you know the answer to the difficult question.