I recently went through this last month with my beloved Persian. She was still eating, using her box, purring etc. but the look in her eye told me that she was no longer happy. She had severe arthritis in her hips. I found this blog post, and it helped me put things in perspective and make my decision. I couldn't stand to see her suffering, and for me, I'd rather put her down sooner than later. Now I know she is no longer in pain

Best of luck in the timing of your decision. It's apparent that you love your little meow. It's certainly one of the hardest things to do!