I completely understand! I went through this last year with my son...and I am so sentimental that of course I had a much tougher time adjusting that he did!
But I would say that after the first few weeks, I started delighting in watching my son grow. I think the kindergarden year is one of HUGE change...and while there is a bit of sadness that comes with that, it is also exciting! Look at how you have raised him and helped him to feel confident enough to go out and meet new friends and get along with his teacher!
We would play a game that we called "Hi, Low, Funny" at the dinner table each night where we would each have to say one good thing about the day, one not-so-good thing about the day, and one thing that made us laugh. Many times it would give us a glimpse into his day and it would spark more conversation (because you know when you ask a kid what they did in school that day, they're going to respond with "I don't know"!)
Also, many teachers have email. I found it helpful to keep in contact with the teacher through email. Our teacher had projects that we could help prepare from home...so that us working moms could still be a "mom-helper"...just not in the classroom. If your teacher has something similar, it might help you feel more involved with the whole process.
I also joined a soccer team with other moms from the school. These moms have kids of all ages in the school...so it helps me learn from their experiences. Also I feel like I can ask them questions about various teachers, processes, etc about the school. You might try and attend the Parent Teacher Association or whatever your school's equivelent is. Then you can also be in the know...as well as help to make decisions that will impact the years that your child attends this school.
A long winded answer to your question, I know. But maybe something from my experience might be of help. My son started first grade this year, and again there was a bit of sadness as I walked him to his classroom...but I also find it exciting to see him having so much fun.
Good luck!
K.