Sometimes the kindest and most loving thing we can do for our beloved animals is to let them go. It is never easy, even when we know it is the right thing to do, and that to keep them would cause them to suffer.
Their little lives are so short compared to ours - and they pack so much love into a tiny bundle that it tears our hearts out when the time comes. But grief is the price we pay for love.
Your life will feel empty and unreal for awhile, and you will never "get over" losing her, but you will come to terms with that empty spot on your lap. Do you have pictures of her? Perhaps you could make a collage, or have an especially lovely one framed, or printed onto a cushion, or a mug that you could fill with flowers in her memory. You have had a wonderful thing - the selfless love of a living creature. The two of you shared more in 17 years than many people ever feel in a lifetime.
But for now, don't be afraid to give in to your grief and pain, and be kind to yourself. It will take time to even begin to feel the reality of your loss - everything will have an unreal quality to it for a while. You'll hear her voice and see her in her favourite spot. You'll find yourself talking to her. It will take time. Surround yourself with people who understand - and if anyone says "It's only a cat. Get another" cut them out of your life. Love is love and knows no age, bounds or species.
Please don't mind me saying this, but one day, perhaps, you will feel ready to let another little love into your life; if you ever do, don't feel it as a betrayal, but rather as a compliment to what a wonderful companion she was. That day might be soon, or it may never come. Either way, you have given and received love and companionship, and have had the courage to do the right thing when the time came.