I remember very well taking my beloved Trouble to the vet just before Christmas three years ago, the vet said that we could keep him going for a few more months, but he would have to have biweekly bloods done and would have to be given pills every day, he was still a purr monster, but had stopped playing, I looked at him, his eyes had started to film with cataracts, and he just wasn't right. I asked the vet what she thought, she said "I think it's time to let him go." In my heart of hearts I knew that, which was why I asked her. She asked if I wanted him taken into the back room, but I said no, so he went to sleep in my arms. It was one of the hardest decisions I have ever made, but I had to do what was right for him. You'll know in your heart of hearts when it is time for Bella. Awful though it is. I am sure your Darling Daughter will recognise this too.
He is in my back garden with a lovely wild white rose on his grave.
As I said before, we are always here for you both.
I think everyone on here has had to make that agonising decision at some point, we do all understand.