19 is a strange age. You're expected to be an adult, yet adulthood is so new. Expectations have changed, and now you have to provide for yourself. It's difficult, especially if you can't drive. You can be mature in other respects, but sometimes these are much less obvious, as they're mental progressions rather than social, so they're not as noticeable. As an adult you're supposed to know what you want to do and have a plan, a steady job, etc. but at 19 it's challenging to have these already figured out and be confident in your choices.
Maturity happens gradually, yet at 19 you're at the age when you officially "know better." It's the major transition from dependence to independence. As you reach more milestones you're treated differently. For example, as someone who depends on others for transportation, I get the typical response of being treated as younger than I am. Not driving at 19 is atypical, which can be alienating and therefore makes me feel like less of an adult because this is a milestone I have yet to reach. In other ways, like not wearing makeup or dating, I don't meet expectations for my age and this gets a weird response.
Other manners reflect age, like appropriate maturity, but where I'm lacking can be prominent at times. It's easy to say that other aspects will improve in time, though it feels like life is moving so fast around me that I need to keep up more to stay current with my generation (I was late to the game with most forms of social media). Knowing that I have time is somewhat comforting, but there's always that lingering hesitation/sensation that I'm doing something wrong. Little things, like filling out paperwork at the doctor's office, or bigger things, like doing your own taxes, are to me equally intimidating, but are necessary evils (that, save for taxes, no one warned me would be part of adulthood). Having a job involving physical labor helps, but it's still a strange situation--one most have when they come home for vacation from school--to have not left home but to have become financially independent.
A license makes a bigger difference than I expected, there's a certain degree of independence you didn't have before a license I imagine you enjoy after getting one. Not relying on others and getting to go wherever whenever and having that control over your own schedule is a freedom I haven't worked up to. It makes me worry that perhaps I'm not as mature as my peers. This anxiety is something I experience with social situations, though I know everyone moves through life at their own pace.