I know that feeling also. I have been with my son full time since he was born. I have him enrolled in kindergarten in September and I'm starting to feel the same way. With my daughter I wasn't with her the whole time and she ended up going to day care so the break was a lot easier. I'm worried about my son though. He has a hard time without me around and I hope he can adjust.
I think the main thing is that we don't let him know we are feeling this way. I also have him enrolled in a day where he gets to see what's going on when I'm around. Also the first day I'm going to walk him to school so then he knows where to go and then the second day he can go on the bus. I hope I have helped you ... if not at least I know how you are feeling.