I'm so sorry that you have to make this decision. You know in your heart what is the best for Roxy and that you will be bringing her peace and end her pain.
Roxy was very blessed to have you in her life for the last three years. What a wonderful soul you have to take in a sick kitty and love her like you did. God Bless you and may God gently bring Roxy home where she can be pain free and happy until you are reunited again one day.