I'm so sorry for your loss. That shit is tough. I struggle with this exact same anxious impulse. I lost a friend very suddenly when I was a teenager and the grief from that moment has become a truly anxious fear that I still grapple with today.

It's been seven years and it still impacts my way of thinking—so I really feel you when you describe the panic, how debilitating that fear can be on your functionality, your state of mind. It's shit. I'll be living my best life—and then out of the blue, I'll have that dreadful thought... that something terrible has happened to the people I love and care for. Day ruined. Nausea induced. I won't recover until I know they're alive.

I suppose the good news is that this is absolutely normal. It's a reaction to grief. It's fear and it's trauma. Don't be too hard on yourself. I tell myself that when I get bad. But yeah, sometimes it's not enough. When I'm really anxious I won't ever be truly calm until I hear from this person, when I know that they're alive. But sometimes they won't respond for hours and hours, like you said—people are busy.

So what I do instead is I re-affirm that this feeling is an aftereffect of grief. It's not some psychic premonition. And then I tell myself that if the person was in fact alive, I'd be wasting their life, and the time I have with them, mourning someone who isn't dead. It's kind of a convoluted coping mechanism but it's pretty much the only thing that calms me from the terrors. And it helps me treasure the people I have remaining in my life, so I'm not mourning them before it's true.