The decisions that you make every single day have the potential to affect you for the rest of your life.
I have suffered (and still do occasionally) from dissociative symptoms. These have been due to an inability to handle a hugely stressful situation that was going on in my life. Dissociative symptoms are impossible to explain. It's almost like you're not completely there. You remember there was this point in your life when you thought rationally, when you could connect with the world, when you could "feel" pain, you could remember easily, and react to events appropriately. Being in a dissociative state is the loneliest feeling that could possibly exist.
Avoid drugs, and be careful who your friends are because like it or not, they will tear you down if they make poor choices and you are striving to making good choices with your life. Live your life trying to have a good name for yourself, but don't try to be someone that you're not and don't try to impress people. Be yourself.
During the ages 18 to 22, you learn cause and effect a great deal, I believe. If you surround yourself with people that you want to be like, you will become like them and you will start to think like them.
I made the mistake of thinking I could change someone who hated his life, had an incredibly poor sense of relating to others, was manipulative, lied, and blamed others for his condition, and was basically hated by a lot of people. I spent years with this person (off and on), and the person that I am today is not who I wanted to be, and it will be a long process of counseling and therapy before I gain a sense of self again.
Please consider what I've said, and I hope that when you reach the last of your years, you won't have significant regrets.