Even with a good career and all the "adult milestones" I don't feel like an actual adult. I feel like I'm pretending to know what I'm doing. Anyone else experience this?
You'll feel like an actual adult when you stop chasing after what you think society expects a successful adult to be.
Not only will that mean you yourself have the self-confidence of an adult and the adult ability to set one's own milestones, but modern day society is pretty shallow and immature and not really design for people to be self-driven and independent (look at celebrity culture, look at how politics use Tribalism so that people react very much like they do with sports tribalism were the stakes are nowhere near as high, look at consumer society powered by marketing using manipulation strategies taken from Psychology).
If you're lucky it might happen when you have your middle life crysis (though many, maybe most, just seem to become infantilised) or as result of some life-changing event.
Once people actually figure themselves out and hence their objectives in life, from the ground up, they often stop trying to be whatever they are told or led to believe are society's metrics for success, mainly because social messaging nowadays is very manipulative and self-serving, so anybody who unquestioningly guides oneself by those metrics is more likely than not just serving somebody else's interests and egos.