I think it is normal, when grieving to think of all these things. The social isolation does sound like the major issue---
I don't know you but your attachment to your cat makes me think that you might get some pleasure and some social contact from doing something like volunteering at an animal shelter (where I am there is one that is all cats) or engaging in some other activity that involves animals and sometimes people. (((((((((((((((hug))))))))))) I once had a cat who went with me everywhere and lived to be about 20yo...still think of her sometimes even though it was years (and a few cats) ago.