There's a lot of things I'd say to a younger me. The most hollow-sounding would be "It gets better."
"No, really. Listen to me.
It's what everyone says and it feels like maybe it's for someone else, that it gets better for other people. But the anguish isn't forever. The loathing, the apathy, the nothingness - the self hate and the gnawing pit inside you, none of it lasts forever.
It doesn't end swiftly, and it doesn't end without effort. But it does. It gets better and you get to see that.
And when it does, you need to tell other people, too. People who needed it when they were younger, people who need it now, who will need it later.
It honest to goodness gets better, and thank goodness you're here to see it."
Self and Art (c) J. Fitzmaurice