Im so sorry about your sad news...I also took cats(cassie and callie) to the vets today, and had them tested for feline leukemia. it was negative thank God. but I am very sad for your cat, and for you, I know that had to be a very hard choice, but, with other cats to consider, you made the right decision, as sad as it was. I know you must be so sad tonight, so upset. its very hard to do something like that. but she knows you loved her, you cared for her the best you could. she knows that, and is happy in rainbow bridge. try not to feel bad, though I know you do.
all you can do is your best, and you have done that.