Empty Nests Support Group
Empty nest syndrome is a general feeling of depression and loneliness that parents/other guardian relatives feel when one or more of their children leave home. Empty nest syndrome has become more prevalent in modern times, as the extended family is becoming less common than in past generations, and the elderly are left living by themselves.
I feel for you and share your mood. I am a working dad, but work does not give the fulfillment, love, joy, and meaning of being a parent (at least for me). Yes, the activities such as PTA, scouts, first day, parent-teacher conferences, homework, sports, room decorating, driving them places, family vacation...all give purpose and meaning.
It is even harder if your marriage is not so wonderful.
The best advice I read was to allow yourself to grieve. Time will do its thing and eventually you will accept this new phase. I am a fan of Buddhism, and the first tenet is the "All things change and this causes suffering." The rest of Buddhism goes on to explain how to handle it. Yes, I am simplifying