I'm so sorry about Mitzee!
Every cat goes thru grief in their own way, very similar to humans. Some can be distracted by additional cat family members, others seek comfort from their humans, others will isolate themselves from others - the variations are endless. I've seen cats that show no outward signs of grief and others that are not normal for a few years. Stumpy, my dog lover, grieved when our dog died and did not come out of it until a year later when we got another dog. Pinky grieved over the loss of his brother for close to 2 years and suddenly just came out of it one day.
To help mine thru the process, I do sit down and talk to them about what happened, acknowledge their grief and tell them I'll try to help them thru it. I tend to over indulge them on the things that I know make them happy, whether that be more play, more snuggles, more friends, etc. It takes the edge off their grief and eventually they adjust.