There's an aspect about the adult life that no one told you about, it's a detail that was overlooked during those days and nights when you longed for independence and becoming a "grown up", and that is having responsibilities. What most of us seem to forget while we're teenagers is that there are in fact responsibilities in life, and eventually one will catch up with the other, and in the beginning responsibility usually comes to bite you in the ass.
It starts with getting a job, even a part time, than maybe you will have to start thinking about paying a couple of bills and still have some money left for having fun with your friends, but eventually you won't have any, and will have to stay at home watching the re-run of pretty woman and eating last night's pop corn. Being an adult is fun, but only sometimes, there is a lot of pain, some of the people you've known your whole life will start leaving you, they age too and time is ruthless, you will also have failed careers, some people will get abortions, other will have to come into realization of their addictions and will have to face them in order to save their lives, but some won't realize that on time, and will stop living at the beginning.
Being an adult is not comfortable, you have to constantly battle the world, save your position at work, fight for that item on sale at KMart, fight for the apartment you need to rent, but not the one you wish you could rent, and even though it's a dump, it's good for now, at least you have a roof over your head right? With being fully grown up you also have people who will start relying on you, and depending on your judgment for some big decisions in their lives, and that is totally uncomfortable.
As an adult you will also find that there is no getting away from certain things, no matter how painful or boring they are, and with time you will start being a more patient and resilient person. As an adult you will find true love and also true heart break, and that sucks... Big time! It is what it is, life has its ways of sneaking things upon us, and the next thing you'll know is that you're going to find yourself sitting in the subway going to work wishing you could now be going to school or summer camp, in those days when you were innocent and thought life was hard.
This is a note to self, it's a note to remember how life is complex and rich, how even with all the downsides of becoming an adult there are still so many great things about it that you will simply forget the bad times, growing up and living one day after the other, learning one thing on top of the other, that is the true richness in life, but understanding these things is the true richness about being an adult.