I am so glad you started this thread as I am also going thru this.
Our family is a mixed group - he had 3 and I had 3 when we got together, after 3 years we had our son. As each one grew up and moved away my heart would crack a little, I would spend sleepless nights and worry but would eventually adjust as I found that they were doing all right and were really not that far away. This year our youngest graduated from high school, he left Saturday for his first job as a summer camp counselor 5 hours away and would not hear of us driving down with him, He will be home in about 6 weeks long enough to get things together to leave for college at least 6 hours away. My head tells me all the things that you are saying, I should be proud and happy that he can go but my heart is breaking in half. It feels like this is going to be one of this things that I will not easily adjust to.