Back to school blues

My son's first day of school is tomorrow, and I'm a total wreck. He is so ready for this and I'm so not. I'm in tears wondering where my little baby went. The tiny prem bub, for whom even five 0's was a bit big, is now four and the size of most 6-7 year olds. I just can't understand where the time went.
It's not even like he hasn't been away from me. I've had him in crèche, and even day care for a while. I handled both situations much better than this. I know I've had a rough year. I had a stillborn bub almost a year ago, and ended up pregnant again quicker than expected. Pregnancy hormones could be playing a part. I just wish I could stop crying over this. I have to drop him off in the morning, and I can't be crying when I do.
I see other people posting about how happy or excited they are for their kids to going to school, and here I am just devastated.