I love all animals but having cats in apartments is just easier. I'm in the process of doing the "hospice" thing for my elder rescue who, had I not taken him the shelter might have put down. I asked him for five years and we're going into the 5th and it's not looking good.
I give you the above paragraph not to compare stories but to give you the background to know that when I say I know exactly how you feel, I do. I'm used to waking up mornings with Dom (Sir Dominic Tattinger III... a regal name for someone who went through such misery) at my side, snuggled into my arm and using my bicep for a pillow.
I miss every single cat I've ever had and I still miss the dog I had as a child. Funny thing is, her coloring (black and white) was very much like Dom's.
My heart goes out to you, I know how you feel. All you can do is to be happy for the time they were with you and how much they enriched your life.
May you know such joy again.