So sorry for your loss.
Dealing with my parents' estates, I found that banks, insurance companies, car registration, pensions dept. and the like fairly easy to deal with, as these companies or public offices are professional and know the drill.
Keeping a "to do list" and crossing out what you have dealt with, and writing down who is phoning you back, or sending confirmation does help.
What maddened me, were the magazines, book subscriptions etc. Long after they had been notified they were still sending letters addressed to the deceased. No amount of phone calls persuaded them to update their databases so they were not still sending letters to a woman who had died a year and a half earlier, thereby upsetting my father all over again.
It was not until I rang for the nth time and said straight out, "Surely you can think far enough to realise that losing your wife after 53 years of marriage is hard enough, without you upsetting my father by persistently sending adverts addressed to my mother.? that these companies got round to deleteing my mother's name and address from their mailing lists.
I pass the tip on for what good it might just do.
IF you have forgotten some or other company, most are very nice about it, if when you remember, or they contact you, you phone up and say, "I am sorry, I must have forgotten to notify you of NN's death last summer."
So do as much or as little as you feel able to, and don't worry about possible omissions.