I know how you feel, my daughter (only child) moved out about 6 years ago when she was 18 to live with her boyfriend, now husband. That was the hardest thing for me to accept. The empty nest syndrome does eventually go away, but your best friend will always be there. She will always need you as a mother and as a friend. When my daughter got married 3 years ago, I was her Matron of Honor and that made me feel real special. She now has a daughter herself, and we are really close. I watch my granddaughter alot and my daughter really likes that I am there to help.
Trust me it will get better. It will take some time to get used to it, but, it does get better. It is hard to let go, but you have to for her sake as well as your own, but remember they always come back.