Soon-to-be empty nester cries daily, sometimes for hours

DEAR ABBY: I’ve been married for 22 years. Our son is leaving for Marine boot camp the day after his 18th birthday. He will be a third-generation Marine, and we are extremely proud of him. He will be stationed in California, several states away from us.
Our soon-to-be 21-year-old daughter graduated from college, and we are also very proud of her. She lives an hour and a half away.
My problem is, although we see our daughter often, every time she goes home, I’m grief-stricken and break down. The idea of our son leaving has me grief-stricken, as well. I can’t even look at him without breaking down and crying. I cry daily, sometimes for hours.
I’m severely depressed to the point that I sometimes don’t make it to work. I have broken down at work as well. I’m heartbroken over becoming an empty nester. I’m crying just writing this. I haven’t been this sad since my dear mom passed away a few years ago.