Middle Age Blues

I am depressed. Our children are grown up and leaving. This should be a cause for celebration since it indicates in my mind that we did mostly the right things as parents. We got what we worked towards. Happy days. The problem lies in that we have these adult children who are so fun and interesting that we want to hang around them. They're going off for adventures and we're staying here. I know that we will survive this and find other fun and interesting people to be with. We have a great group of friends, jobs that we enjoy most of the time, outdoor pursuits, and a house that needs renovation. We have 2 out of 4 kids who are close by. Everyone is happy and heathy. So why is this so difficult?