Sorry you have lost your much-loved Lucy. We had much the same with our little cat, Sherlock, in January 2013. I stayed with him to the end also. We all missed him so much and eventually we just missed having “a cat” around our home - but we had a building project going on, so it wasn’t fair to bring another cat into the chaos.
In December 2013, we finally finished our project and Tom came to live with us. He was a rescue cat, who had been left behind when his original family moved on; he lived rough for two years, being fed sometimes by a neighbour, who then took him to the vet when his eye became inflamed. After treatment, he spent a whole year in a foster kennels, as he is black (not a “cool” colour for cats!) and very fluffy (people can’t be bothered with grooming - but Tom loves it!). We brought him home and he is now an established member of our family. He’s so happy that we know it was the right thing, to bring another cat into our home. Memories of Sherlock are sweeter, for having given a home to another lonely puss, who needed the love we had to give him. One day, Tom will join Sherlock and they will maybe play together - Just like Lucy and Louie!
I sympathise with your loss - I know how devastating and agonising it is, to have to make such a decision. It was the right thing to do and you didn’t desert Lucy at the last, but stayed with her. She is now at peace, so I hope her peacefulness will envelop you and that you will soon feel able to let her go. Perhaps you may feel, later on, that you can offer a home to another sad and lonely cat, who is waiting for a family.