As I grow older I realize one of the most important part of the "being an adult" experience is the ability to just "let it go". In not so positive words - it is literally getting used to the feeling of helplessness.
I had an amazingly protected childhood. I generally got what I wanted and I grew up with this attitude that I can get what I want if I work hard towards it. So, my childhood was all about, setting a goal and working towards it. Then adulthood happened and as I grow older I realize - you can't achieve a lot of things that you think you can. It's not that you are not putting enough effort or you are not focused, but there are a billion things beyond your control.
As I sit and ruminate on my present state, I think the most important lesson that I have learnt over the last decade or so is that - you have to try hard for whatever you want to do or achieve. But if you can't you have to gracefully let it go. The "being adult" part is the letting it go and moving on part. The more you can do it faster, the better your life is going to be.