It's hard! But on the plus side, you watch them grow and it's amazing:)
My kids are 22, 21, 20 and 17. I think of them as just that.. my children.
But when I see them in their own spheres (21 yr old ds is a support worker, responsible for a couple of highly volatile chaps with schizophrenia and learning disabilities.. 22 yr old is a 5th yr medical student, 21 yr old 3rd yr nurse student)... I realise that the outside world sees them as adults! This is mindblowing to me 
Tonight I have DD1 and DS1 (who still lives at home) with me.. they are entertaining, funny, discussing the Scottish Referndum and they are just as loving, cuddly and gorgeous as they were at 5. The separation period of 18+ has settled back into a much closer relationship ..with me and Dh and with eachother.
DD1 will be a doctor in 10 months time... that actually blows my tiny mind somewhat... a real fully functional adult! But she still comes home when she can, still cuddles me on the sofa... and it is FINE.
You let them go, and they come back:) My girls have been at Uni for 4 and2 years so far and it just makes the times together more precious. Hang in there!