Adult is just being 18+ years old. What you may be looking for is how to mature as an adult. That’s done by trying, failing, and learning over and over again. You’ll always have some fear of new things, but you eventually learn how to bounce back from failure to reduce the fear. As you get older, you’ll lose the support of family because they die. As that happens, you’ll learn to fend for yourself. You will not mature if your are doing the same old stuff because it’s comfortable.