High school is just a preparation stage before the start of your real life, please dont give a **** about the opinions of people or "friendships" with people who you will never see again in your life.
I'd tell myself "Stop trying to please others and please yourself. Put yourself first because as you grow up, people will take your kindness for weakness."
Don't let people get to you. You'll accomplish more when you let go of your fears; everyone enjoys your company. You are going to mess up a dozen of times, but you have to understand that's the price of life; trust me, you'll learn so much more from your mistakes. And make sure you hold on to your family: You never know who will be gone from your life next, but don't cry because they'll always be right there with you.