1. I know you feel as if you’re all grown up, but you’re not — sorry. You may be supporting yourself (just barely) and be responsible for yourself (not always wisely) but you have a long way to go: you have (most of) your whole life ahead of you. Try to remember that.
2. I know you’re worried about making the wrong decisions — sometimes paralyzingly worried. And it’s true, you’re going to make a lot of bad decisions. But then eventually you’ll start making good ones. And even the bad ones will teach you something. (Keep this job or quit it? Keep this guy or quit him? Work harder at this or harder at that or ditch both things? Etc. Either way. Seriously. Stop fretting and pick one.)
3. I’m not suggesting that you stop trying to figure out the “right thing” to do. Only that you limit the amount of time and energy you’re spending on the figuring and fretting, and that you trust yourself more. You can trust yourself.
4. And I know you’re worried about not being good enough at the things that matter most to you. Well, you’re not, yet. But you’ll get better. And the whole idea of what “good enough” is will be a moving target anyway. So just keep trying. Don’t waste your energy doubting yourself. (See “trust yourself,” above.)
5. And, no, I’m not suggesting that you stop working at getting better (this message from the future is only a guarantee of that future if you keep doing what you’re doing — not if you use it as an excuse to sit back and wait for better days). All I’m saying here is: keep up the good work, and stop worrying about how you’re not yet getting the results you want, or how you’re not doing exactly what you want to be doing for the rest of your life. I can tell you with great certainty, here from the world of your future, that one day you will be doing exactly what you so long to be doing.
6. And please knock off the grim daydreaming about a future that looks exactly like your present (except that in the daydream, you’re old — and that’s the only thing that’s changed). Everything will change. Whether you want them to or not, things change. That’s how life works.
7. You’re going to stop smoking in five years. So why not do it now? Might give you a few extra months somewhere down the line.
8. Be nicer to your father. He’s just trying to understand your life, he’s worried about you, and every single thing he says comes from a place of love. No matter how infuriated you get — no matter how wrong he is sometimes (okay, often) — hold your temper. Because 35 years from now, he’ll be gone, and one of the very few regrets you’re going to have about your life is not being more patient and kinder to him from ages 15 through 29.
9. That girl you just befriended? She’s going to be your most trusted friend for the rest of you life. I’m just sayin’. (So keep this in mind: you never know who, among the people you know right now, are going to be the people you’ll know, love, and be able to count on, twenty, thirty, or more years from now. Best bet: proceed as if all of them will be. That way you can’t go wrong. Pay close attention to the people you care about: really listen to them when they talk; remember what they say — and what you do together, how you feel when you’re with them. Pay close attention to everything around you, in fact. You’ll want to remember all this. That’s a piece of bonus advice.)
10. Don’t waste energy regretting what you’ve already done (“I should never have dated him — that’s six months of my life I’ll never get back”; “That dead-end job sucked eight hours a day out of my life for a year”). Everything you do, even the mistakes you make (bad relationship decisions), even the things you’d rather not be doing (a job you hate), even the thousands of pages you spent thousands of hours writing, then put in a drawer, will teach you something you couldn’t possibly have learned otherwise, prepared you in some way for what’s ahead, or just expanded your sense of how the world (or love, or friendship, or a novel, for that matter) works.
11. I promise: you’re not going to be disappointed in yourself. If you could walk into your future and meet your much older self, you’ll be pleased, I’m pretty sure, with the life she cooked up for you.
12. And before I go — listen: you’re not only better, smarter, and more resilient than you think you are, you’re also prettier. Try to enjoy your youth. It doesn’t last forever. 
