My teenage self was traumatized. A childhood of irregular parenting, threats, and molestation will do that to a person. So will bullying at school. I had experienced all, and I became convinced of my unworthiness. It didn’t help that I was overweight, highly intelligent, and hopelessly awkward.
High school was a little better, with a reprieve from the bullying. I didn’t have any close friends, but at least I didn’t have active enemies. And I discovered extracurricular activities, like theatre and choir, and I did well at them. Still, it was a hellacious time for me. High school is filled with popularity contests (I was the polar opposite of popular), first loves (mine was unrequited), and future plans (at least I had that down — I wanted to go to college more than anything).

I survived high school, and then I went to college, where I finally found people like myself (nerds). I never felt like I truly belonged anywhere, but I found places where I fit in. College changed my life.
If I had my teenage self sitting in front of me, I would tell her that things will get better, that high school wasn’t life but a road block to get over. The high school experience isn’t real — going to prom with someone didn’t mean true love, and even true love dissolved in weeks (except for my unrequited crush, who married his high school sweetheart). The popularity will fade and have no meaning in one’s life. There are things that matter more, like finding one’s true self and navigating the world. I would be tempted to give her one glimpse of her future life, but I wouldn’t do so, because I want her future to be a surprise to her. A pleasant surprise.