It's tough when cats start to slow down. They can live happily for many years after being a bit slower, they just have to be a bit more careful, they aren't young kittens like they used to be. :)
I hope you have many more years with your kitty. It's really hard to lose an animal, especially one who has been with you for years like that. My boyfriends family recently lost their cat in a tragic accident and it has been quite an emotional time for everyone. :(
Our pets live such short lives, but in a way it's amazing, we can be a blessing to them. We are a stable person in their lives--often from as early as a few weeks old until the very end. We are their security, their love, their home. And we are with them until the end, and I know they appreciate that in one way or another. It's hard when they pass on, but at that point they are no longer in pain-and never have to be in pain again. We have to suffer with them until they no longer have to suffer any more. It's hard for us, but remember you don't have to be sad for your cat. Especially when they get to pass on peacefully. What an amazing life for a cat--a family that loves and feeds you and plays with you, taking care of you from now until the very end. Living your final years feeling safe and secure with your family. Growing old and finally going to rest, never having anything really to fear in life or years of extreme hardship. A great life. That cat isn't sad at all, and I'm sure if cats could talk she would tell you not to be sad after she leaves. It's a moment to remember the life you and your family gave that cat. Remember it fondly and proudly and keep the kitty in your heart when that day comes. It isn't easy, but remembering these things does help.
And you may not have to deal with this for many years. You may just have an older cat who just wants to take life slow and comfortably. Just continue to give the cat the best life ever and continue to be a miracle in her life. Lots of cats have no homes or terrible ones. Your cat could have been one of those cats had you not taken her in. But she was lucky. She got a family and an owner who got to grow up with her. :) That's special.
I know I will never forget my pets who have passed on. I cried a lot for each one. I still get sad sometimes, but I try to remember the happy memories and the fact that I was the best owner I could be for my animals, and that they were happy pets. :) I find my own ways to remember them and pay tribute to them. Sometimes when I write stories I will include a character who has a pet who just happens to be similar to one of my old pets. For my betta, I carved a little rock that sits in my garden, so his image still is dancing among the beautiful plants like he would have enjoyed doing. There are lots of ways to help us heal, but most of it is just time.
But again, don't worry about it yet. Right now, just worry about loving your cat. :)