Of course they are huge milestones and I bet your child cannot wait to be a big girl and be in school. It is ery exciting for her.
Please don't let your fears be known to her. She sees this as her growing up and as a parent, you see it as her growing up and before you know it she'll be going to college.
We have to learn to let go and learn to love every stage. My daughter (only child) will be a senior in high school next year! I am so excitied for her and of course, part of me is torn because my baby will be heading off to college in the fall of 2013. It is a part of life we have to deal with.
I would advise you to get involved with volunteering at the school. However, if your daughter is the clingy type, you don't want to be volunteering in her classroom because she needs to be in that environment and learn.
I also substitute teach and there are some moms who still bring the children to the classroom in the 1st grade and hang out until way past the morning bells, come back for lunch and one even goes outside for recess. It is obvious that this hurts her daughter because instead of playing with friends, etc, mom is ALWAYS there.
Hang in there with your emotions, you are normal. Just don't let your daughter pick up on your sadness... make this her adventure.