I think it's very common, after someone we love has died, for us to fear that others we love will die, too. This is especially common when someone dies and we are very young.

My dad died when I was well into adulthood, but still every time I can't get my mom on the phone, I'm terrified that she's died, too. It's a burden.

Talking about the fear, and talking with your husband about how much you are afraid of losing him may help some of this abate. It takes such time, doesn't it? Always feels like too much.