Here is what I would say:
1. I understand how you feel. You want to protect your child because you want the best for them.
2. If you do not let your child make his/her own mistakes, they will not learn how to fail. This is an important lesson because they will fail at something. When they do, hopefully they will have learned how to pick themselves back up.
3. You have to let go, or they may rebel.
4. Look at life through their eyes. You were young too, and may remember thinking you knew everything!
5. Sometimes when you let kids have more responsibility they will surprise you and step up.
6. Relax! I have two young children, and would love to have a day where I don't need to change a diaper, clean up vomit, or a spill (don't get me wrong, I love being a mommy!)
7. You may have a better relationship with them if you respect the fact that they are an adult, even if you still see them as your baby.
8. Take a breath. You need to respect the new boundaries since they are older.
9. Don't let them see you sweat. Even if you are worried, accept their decision. Acceptance does not mean you agree.
10. Get a hobby. It may help to busy yourself while you are waiting for them to crawl back to you telling you they were wrong. Some kids are more stubborn. It took me 10 years! It wasn't until I was really on my own, even until I was a mother of my own that I realized how right my parents were.
They never once rubbed it in my face. Now, we are very close and have so much respect for each other. I spent the first 1/3 of my life driving my parents crazy, but I will spend the rest of it trying to make up for my youth!