At 18, you are on the cusp of adulthood. It is normal to be evaluating relationships and your role in them. It is normal to struggle with how to maintain relationships when you find yourself in conflict with others in your life. What isn’t useful is deciding “You can’t fire me, I quit.” Doing that, whenever there is emotional risk, will leave you alone and lonely. One of the biggest challenges in life is learning how to get along with other people, especially when you are in serious disagreement.
Although you say you don’t care about people you should care about, I’m going to venture the guess that you are in fact a very sensitive guy who protects himself by distancing from others. There are better ways to protect yourself from hurt; ways that preserve the relationships when things get tough.
I urge you to see a counselor to help you figure this out. You deserve to have a life that includes friendship and love.
I wish you well.