My heart goes out to you. I had to tell my mother every day that my dad had died. She had dementia, but part of it was denial. She didn't want to believe it. So, I'd have to tell her again and again. It was awful.
Please understand that there's not much you can do but be gentle and keep telling her. Try to reassure her that he's in a "better place" if that fits with her beliefs. At the very least, you can tell her he's "out of pain" or whatever seems positive. It's the truth and it will help soften the blow.
This is likely harder on you than on your mother, but it does feel to you like you are opening the wound for her over and over. Yet, she needs to hear it. Eventually, it will sink in and/or she will forget about him, which is painful for you, too.
Often, this leads to the death of a spouse closely following one another. My mother lasted another five months. I do believe that they can die of a broken heart, or that they just figure it's time to "follow". Of course, they are in poor health, anyway, but as the news sinks in that their spouse has died, many do follow. I am not saying this to make you sad, but to prepare you that this is a possibility.
It doesn't always happen. Some people move on and forget the pain. Whichever way it goes, this is a horribly hard time for you. Please try to take care of yourself,
Carol