Please accept my condolences at your loss. My own dog passed away while I was at university. We had grown up together, and he slept in my bed from the time he came to us as a pup. I still chuckle over the things we shared and did together. In time, you will remember only the love you had, and you will be able to look back with cherished memories. You should feel no guilt. You shared love, your life, and your home for 15 years. You gave your cat a life to be envied. You will carry that love in your heart forever. Grieve as you must, but leave yourself room in the future to enjoy the memories of the time you shared.