If I had a chance to leave a letter for my self. This is what it would say:
You may think you have all the answers but you don't. Listen to your parents and spend as much time as you can with them. You're daddy loves you and you love him but he won't be around forever. You have the best grandparents in the world enjoy them while they are still here. Don't rush into anything with a boy. You're about to meet the love of your life. No, you won't die alone. Listen to your daddy about boys and stay in school. It may not be fun but try your best, it's worth it. You're best friend loves you. Make sure you keep making time to spend together. And remember she's your best friend for a reason, that means you can tell her anything. So do. She won't judge you, she loves you. Keep going to see you momma every chance you get, she loves you and we'll miss you if don't make time to go see her. Your sister and your brother love you, they just have to figure out themselves before they can know how to show it. You may think you are so grown up right now and that you know everything about the world, but you don't. Life isn't about the internet or drama. Listen to others but don't let them take advantage you. You are your own person and you don't need people in your life who don't love you for you.
Life can be amazing if you just learn to not sweat the small stuff and don't let others steal your joy (and lesson you're still learning).