The first step for me towards becoming an adult was realizing that the world was bigger than me, and my friends, and my school, and all the things we cared about in that little world. Perspective. Once I was able to see over the fence, so to speak, the little things which made me sad, and ego, and all of that just didn't seem to matter anymore -- the sense of angst that I had as a teenager began to go away. I saw angst for what it was -- a desire for attention coupled with a self-absorption to justify it. And why did I want attention? because I found the little world I inhabited to be empty and meaningless, and I think a lot of teenagers feel angst because they feel the same way about their own little worlds. But the idea of the world being only what you know of it is delusion, and it's startling to realize that. You know, silly things like, "Why should I do homework? I don't see any reason why! God this is so stupid!" once I was able to accept that the world was bigger than what I knew, the fact that I couldn't see a reason became a non-issue, because I realized that I couldn't see much at all. The world consists primarily of things to learn, and the more you learn, the more you realize you don't understand. When I was caught up in the claustrophobia of thinking I knew everything, I was completely unable to see that.
Anyway, I'm rambling and this thread belongs in Life Issues. I suppose my tl;dr is to relax and be open to new and unexpected things. It takes a lot of planning and then seeing those plans knocked down to realize that life is a wild ride and you can't control it.