I've gone through empty nest syndrome several times...when my youngest, the only girl, left for college in a neighboring state, I went into therapy...it helped me so much.
She's decided to stay up in Wis... but she visits often, and calls a few times a week.
My second son has never come home to live once he graduated...he's now in Baltimore with his wife working on his PhD...doesn't call much, only see him 1-2 times a year.
The other two sons live in the area...each has moved back home at one time or another...unfortunately, once I'd gotten used to an empty house and actually enjoyed it.
We now call our house a bed and breakfast.

I can only tell you to allow yourself to grieve for awhile...then it's time to find out who YOU are...we do live through our kids, and their friends...it actually makes it harder when they fly away...
that's why you need to find out about you now...take classes, join a bible study group, book club, photography, computer classes, all these things that will make your brain grow...
good luck...