Write a letter to yourself at seventeen.
When I was seventeen... it was a very good year... la la da la la la... la la la La da da da ...
Here's how you'll know it's me. You had a hand-drawn map of Prydain up on your bedroom wall and used to stare at it, willing yourself somehow to appear there, a la "Voyage of the Dawn Treader".
Oh, my dear, sweet, innocent self. I have so many things to tell you.
First of all - it turns out okay in the end. Maybe not exactly the way you expected, but really okay, no matter what.
Quit smoking. Now. You have only just really gotten into it, and it's going to dominate your life for the next twenty three years and it shouldn't. So please, trust me, you really don't need this.
Right now, you want to be an actress. Don't be discouraged. Go for it. You'll always regret it if you don't.
Leave home. Your father is poison, and you don't need to be afraid of the world. Don't listen to them. Do what you want to do.
Robert is gay. Get over him. Dougald would probably be a much better bet. You might also want to explore the unconventional. You have some scars, some BIG scars, from earlier experiences that you probably need to get dealt with. Don't be ashamed or afraid to get professional help.
When you decide to go to university (that's if you don't seriously follow up on acting), do English. Don't go into Classics - it will get you nowhere, and you're going to end up in English anyway so bow to the inevitable and get there faster and more painlessly.
If, however, you decide to go into Classics, go after Bert. He's a Good Thing, and he loves you. Don't let pride get in your way. Be honest, be vulnerable, don't worry about being hurt and it will all work out.
Oh my. Follow your bliss, my dear, and everything will be fine.