How do you let go?

My daughter who is going to turn 17 years old in a week keeps talking about how she and her best friend are going to California when school is over in 2 years. No one is going to stop them and they are old enough to make decisions. So I just go along with it but I am scare to death for her to be alone and that far way. I can't just get in the car and in 30 minutes save her if something happen, we live in Texas. She has always been the one I worried about the most, she is very shy, scared to touch the stove she might get burned, at restaurants she always need someone to go to the restroom with her. Its just if anything happen to my baby I could not live with my self. She is the older, I have 13,15 year old girls, but they are very out going, cook when they want, walk to the store on there on. Two years is very soon, and would if they up and don't wait to years because they are so excited? I want her to have an exciting life and live and learn, I always thought of her as my delicate butterfly, for gods sake she never even had a boyfriend, but her sisters have. How do you let go?