"If I could tell someone younger something, what?"
Enjoy your youth. Enjoy the time you are able to spend unencumbered by the expectations and realities of the adult world. Childhood can be absolutely wonderful if you give yourself the opportunity to learn, explore and play. I think it is equally important for adults and yet it's so easy to be caught up in survival mode as an adult and trying to navigate a world where your privilege determines much of what you are able to accomplish.
I had a difficult childhood at times but even so, I wish I had spent more time being child-like, carefree and playful. I took life too seriously from the get go. Not to diminish academic achievement or being a determined student, but I feel that I robbed myself of many opportunities and experiences with others/friends and learning about who I am. While that is a lifelong process, being extremely withdrawn and afraid of life didn't do me any favors. Often times I find myself wishing I could rewind that clock and have an opportunity to experience life in different ways. Even something that might not necessarily have had a huge impact but may have improved my life in some respects would have been graduating and getting my GED early so I could pursue college or life earlier rather than feeling stuck and attending high school without any friends.
And don't be so hard on yourself. Life is learning. Making mistakes shows you you're making progress. You always have the opportunity to learn from mistakes and improve. It's a huge part of life. Let go of what you cannot control and anything pertaining to you is often within your control. You may be frustrated or distraught by circumstance but how you cope is within your power. Empower yourself. Follow your intuition.