Yes this is inevitable for us as parents, but it is also a time to celebrate his growth and make necessary changes in your own life.
You have good reason to be proud of your son as he takes on the responsibilities of an adult. Your work now is to take on the responsibilities of being a capable and self-reliant father by dealing with your isolation. First and foremost you need to be connected to others. At the one end I would encourage you to join a support group for those in the armed forces and you can get that information here.
Don’t wait. There is help available and the time to get it is now.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral