I can totally relate to your feelings having seen our three children leave home to go to college. The summer before the first one left was so exciting for me. We spent a great deal of time shopping for linens and things she would need for her dorm room. As the day to leave approached, I was just as happy and excited as she was until.........we dropped her off and I saw her wave from her second story window....WOW----what a wave of emotions and reality set in. I cried all the way home (and it was a four hour ride......)
It was hard for awhile, I won't argue with you. At least, when the the next two went away I was better prepared. I always said that communities should have support groups for mothers of children going off to college.
But, I was and am very proud of the people they have become. I still keep in close contact with all three. Seeing your children grow up and make choices about life partners and jobs and such is exciting! You'll like that part. In the meantime, keep busy and please know that you are not alone and that your feelings are quite normal to boot!
(as a postscript, I have to add that for various reasons each of the three came home for extended visits that lasted months----the boomerang phenomena of this generation!)