It is so hard. We took my dog - it was the right time, he had heart failure, he couldn't lie down properly because his chest cavity kept filling with fluids. I thought he'd go three separate times that week, but whenever I called DP to come and say goodbye, he perked up as soon as DP was almost home and would have his meds and settle down.
We took him in the evening and although he was struggling at home, and lacking a lot of the energy and personality that made him him, he sparked up again as soon as he got in the vets. He peed on their Christmas tree, barked at all the cats, couldn't have looked anymore full of life. He had a good fuss from the receptionist.
I sobbed so much whilst they were prepping him that they stopped and said I could take him home, if I wanted, if I wasn't ready. I didn't, because he was ready. I don't think I ever would have been. DP's mum cleaned away his bowls and leads before we got back but I found it really hard having them and him gone. It was hard.
It's a bit less hard, now, it's nearly five years later. His anniversary is soon and we'll eat something he loved that day and look at photos of him and generally remember him, and it will hurt a bit but I'll know that it was the right time for him.
It sounds like it was the right time for your cat, too. She'll have known you loved her and you were there with her, and that's a nice way to go.There's always guilt, unfortunately - guilt that it was too soon, that it was too late, that you couldn't stop her getting ill. It does fade. Look after yourself 