Develop, Don’t Change
You've crossed another year off the calendar and started your life as an adult. By no means am I an expert on what it means to grow up (just ask any of my friends, they'll tell you I haven't!) but I do know a think or two about becoming who you are, which is exactly what you're doing every day you're alive. Without sounding pretentios, let me help you navigate this transition.
First of all, know that you don't magically become an adult once you're 18 years old. You're still learning, still growing, and still deciding who you want to be. in reality, we never stop doing any of those things - we are always becoming who we are. Pay attention to the world around you. Do what comes naturally and don't worry about fitting in or who will or will not approve of you - if they don't like you, find those who do. The world is full of diverse and interesting people, and you can find a place in it without sacrificing yourself.
Always be learning. Always be learning. Whether you're in a college course or a trade school or first-day training for your new job, be that dilligent student at the front of the class. Pay attention, take good notes, and most importantly, never be embarassed to ask questions. The same goes for social situations and personal pursuits - pay attention to the people around you, how and why they do what they do. Learn from their actions and the responses to their actions. Be mindful of cause and effect. And again, never be afraid to ask questions. The person you ask will feel complimented that you turned to them for guidance, and you'll learn something new.
Go with the flow. It may sound cliche, but many times it's all we can really do. Learn what you can change and what you cannot - influence what you can and accept what you cannot. There is much strength and much wisdom in recognizing when you are powerless in a given situation and taking your hands off the wheel to let life play out as it may. You don't have to push for those sorts of things to happen - they will happen with or without you. You just have to respond to them in a constructive way.
Finally, as the title states, you don't change, you develop. Being an adult doesn't mean discarding the person that you are. You are not a catterpillar expected to cocoon up and transform into a butterfly. You are a human, and being human means transitioning gradually, one piece at a time, into a more evolved version of your current self. You're not tearing down an old house and remodelling, you're starting with a foundation and building upon it until it's comfortable enough to live in. Don't become someone new, become a stronger, smarter, and more well-rounded version of who you are now.
And remembers: nobody has the answers, even if they claim to. We are all figuring out our lives day by day, learning and developing right alongside you. Whether you're 18 or 80, we never stop developing, never stop growing, never stop becoming more of who we are. This isn't a new journey for you - it's an awakening to a journey you've been on all your life. You've got this.