My third is about to graduate HS, but is taking a year off so I have that time to ease myself into him leaving.

For many parents, it's not just missing them, it's that it changes the family dynamics forever. You go from three to two kids at home, (or two to one, etc etc) and often they're not coming back. It frequently takes parents by surprise when they seem to be 'grieving' for their departed kids, but it is a kind of grief - for a family life that will forever change.

For anyone in that situation - it helps to focus on how excited most kids are to be heading off to college.