Try not to get too upset by thinking you'll never be as close again. It's amazing sometimes how things can turn out.
I was just like you when my son went away to university, he's my only one & we'd always been really close. Managed to say goodbye with a smile, but cried all the way home!
After he completed his degree, he came home for a year, I felt highly honoured!
Then he spent another year working in the Middle East, came home after that for a couple of years, then bought a house just a half hour drive away. He's married now, with a brand new baby, & I see them lots. His wife spends more time here than he does! She's become the daughter I never had.
It sounds like you & your daughter are really close, you should be proud of the good job you've done raising her to become a friend. Whatever the future holds you'll always have that.