Now I am not going to say I know everything there is to know about design, about the professional world, or about much of what adult life is. But what I am going to say and expand upon is that the small individual spaces we are shown in college and throughout academia are falsehoods. They are not bad, presa, but they do little to actually prepare us for what the world is.
Understand business. Broad statement, yet still heavily true. The world of design and marketing are the same coin. The only change is how the metrics are measures. Return on investment versus return on interaction. Audience acceptance versus A/B Testing. One cannot function to it highest level without the other, and to know both is going into battle not armed with a sword OR shield, but with all weapons on hand. To understand not just who the design or demographic is, but why it is happening this way, why the avenue it is being funneled through was chosen, and how that can lend itself to more expressive and interactive pieces. It is not the most exciting thing, but to know the language and terms of the marketing world as a designer is akin to being able to speak Latin when conversing with any romance language. You will not know it all, but you will be able to fill in the gaps.
No one knows what they are doing. Imposter Syndrome is real. Being a designer (or in my opinion, an adult) is more of a dart throw in the dark then any planning. Not to say you can wing it 24/7. Learn to save, understand bills and APR rates, invest in the future while paying for the present. But know that no matter how old someone is and no matter how good at being a designer, a professional, a CEO, or an adult someone appears, no one has all the answers and more then likely they are winging the bigger things. There are no sure bets. There are no completely encompassing answers.