I'm so sorry to hear that your beautiful tuxie girl Mittens has gone to the Rainbow Bridge.
She lived a long wonderful life with you.
I know how you are feeling right now since my sweet girl Samantha went to the Rainbow Bridge not too long ago. She was 20 years old and was blind and deaf and had many other health issues.
It is hard to say goodbye after so many years together, try to remember the good times you had with her.