I'm so sorry to hear this johnnaboo
I lost my childhood cat when he was 17. We had the same birthday. He was
mine.
It was three days before I was going to come home from school and he threw a saddle thrombus. He had been on the decline for a while but nothing that couldn't be attributed to just being an old man cat. Still did all his normal activities and what not. It hit me really hard and three years later I still have a hard time talking about him without crying.
It's okay to feel upset. Even for years after. I'm upset and hurt and grieving because I loved him. There isn't a cure for grief because reducing grief quickly means letting go of some of your love and that's simply not an option for so many reasons. It gets easier as time goes on. Not easier because I love him less, but easier because I've gotten more used to not having him around.
I encourage you to spend some good time with your kitty. Take pictures. Take videos. Pet her and hold her on her lap and share your love with her as much as you can before you let her go. Give her the good food she's not always allowed to have. Set her on the heated blanket and let her get cozy there all day. When you do let her go, get paw prints or a lock of her hair. There's nothing wrong with saving pieces of her to remember her by.
One last thing I will add to this rambling post... I had dreams about my cat for years after he died. Always the same one. He would be back again and in my arms and I was told, very clearly, that if I ever let him go he would go away forever. I could have him as long as I wanted so long as I never let him go. I carried him everywhere, to school, to work, in the car, to the movies, in the shower, around the mailbox. I could never bring myself to set him down and I had this dream probably once a week for at least a year if not a year and a half. I always woke up crying after I had it. Finally in one of the dreams... I don't know. I was ready, I guess. I had carried him long enough and knew carrying him everywhere was bad for both of us. I set him down, and I haven't had that dream ever since then.
It's okay to let them go. It's okay to set them down.