It is so sweet you remember your niece being little and those fun times. I often think back to when my niece was little and the funny things she would say - my son too, they are both grown now. Sometimes I like to look back at old photos, do you do this? I have special ones framed and on display so I can look at them any time I like.
I think memories like this, your nana, your great aunts, your cat, can help shape who we are - in that way although things change and we lose people they never really leave us.
It's natural to worry you could have done more for your cat because we feel responsibility towards our pets but it's quite likely his time had come and he had a happy life with you.
It is difficult now I agree, it's hard to put things in perspective with lockdown and restrictions, it's left us with more quiet introspective time, maybe too much. Are there people you can reach out to and chat with now?