Hey, pal, I am very old and I have lost many special pets. It really sucks! I can only hope that you had a pic to keep.
As you said, you go crazy and grieve a bit, but the best thing to do after a period of adjustment is to get another pet. If it is another cat you want, then find a bunch of kittens and allow the kitten to choose you. You will have a loyal friend for the extent of its life.
If it is a dog you want then I would caution you to respect the area you have available. DO not get a big dog and force it to live in an apartment with no outdoor life.
Small dogs do OK in Apartment life as long as they get to run and walk every day.
I feel empathy for your loss.