Grown Teens Leaving The Nest And Learning How To Fly On Their Own

As a mother who has had children at home from the time I was 18 until my mid 50's it's very hard letting them go to fly out on their own. But my youngest who left less than 2 years ago has really done well for herself, and yet I still find myself trying to give her advice as if she were still at home. It is so very difficult for me to stand back and watch as she goes through life's struggles.  I enjoy seeing her good times but when I see a potential downfall coming or seeing her go through something harder in life, I just sometimes find myself blurting out my opinions. I need to let her experience life and find out on her own, but it's so hard! I find that when I catch myself giving advice, she tells me that she remembers that I already taught her that and that she knows!  I have to apologize to her because I KNOW that she KNOWS, but for me, it's been very difficult letting her go. She doesn't live near me anymore but since she has been in the Navy, and married a Navy boy, she had to deal with new experiences and she calls for my advice, which is good, but I feel I need to stand back a bit more and allow her wings to spread and learn. Why does it have to be so hard when our children grow up and leave home?