I know many who have felt the same way when their kids went to school. I was a bit more celebratory of it. It is so good for them, and was good for me mentally as well.
Yesterday my kids had their first day, and it was a bigger day for me than them. My Son drove to school for the first time, a JR. this year. My daughter started her first day of high school. It is exciting and nerve wracking for them and me both. But each year they are growing into wonderful people who will soon be indepenedent, and off to college.
It all goes by so quickly, I just try to enjoy the big moments, and remind myself that soon they will be gone.