Becoming 18 does not make you an adult. Adulthood comes from maturity, and maturity is taking responsiblity for your life. You may not be an adult when you're old, or you may be an adult before you become legally allowed to drink and drive.
As for what I would tell to the 17-year-old me...
I know you're scared. Scared of everything that you encounter, feeling that it may threaten you - your autonomy, your sense of self. I just want you to know that encountering things head-on is always scary, and it will hurt, but as you already know so far - and are too comfortable to admit to yourself - it only hurts for a while, and the results remain.
You don't feel like your own man. You won't until you'll learn to take care of yourself, whatever that is that requires taking care of. So take a chance; take History for your ЕГЭ* and apply for a university outside of Kemerovo. Let me tell you why.
If you were still me, you'd go to aunt Lena to Mezhduretchensk when you're 20. You'd meet your older half-brother, Lyosha, there, and he would tell you just how much hard work may be of benefit to you. He'll be working in Saint Petersburg by that time, fixing comms for МТС* on top of the roofs, even climbing the Winter Palace once for business. He'd tell you about 13-hour workdays he's been toughing through before he got to such a cushy position. Did I mention he makes eighty thousand rubles a month? Oh yeah. I know this kind of material reward will attract you. You want good things, and good things cost money.
He'd tell you about how he did that well through such hard days of labor: he did football* in the university, and it got him to a great physical condition. On your way back home, you'd tell your father that you want to do as good as Lyosha, and that university is now the best chance you see for that. He'd agree, and your parents would gather money for that.
Now, if you were still me, you'd pick up that History, but slack off studying before it's just a month till the exam, and you know you don't know jack about History. It's a miracle you would've done as well as you would've because of that big brain of yours. You have always been a clever fellow but failed to apply yourself. You tell yourself that you don't need to prove yourself to anyone, but you lie about exactly one person you want to impress: yourself. I know how much you like doing things because of how cool they are, and you haven't learned that well enough by now, but you feel really good after you impress yourself with whatever feat you find really cool to achieve.
You still have six months till the exam. Tell them that you take history instead of Literature (which you'd start studying as late as you would have History; take the hint) and start studying right now. You don't have to hurry, but by all good and holy, don't you slack off, either. After you do study and have those fifty points, apply to the Novosibirst State University. You already know why, and if you'd still be me, you'd visit Novosibirsk again for your eighteenth birthday, and you'd spend a whole day there - starting at getting up at 4 AM to get to the 5 AM bus, walking the whole time you're in town and getting back home at 1 AM next day - and you'd love every damn second of it. Hey, maybe you should do that again! You'd've gifted a Mars bar to that cool girl in the Аура* foodcourt, and now that you know, I urge you: take a chance and actually talk to her. She seemed a bit older than you, but fuck it: better to know than not to.
Anyway. Apply to the NSU. I don't know if you'd like it, but by goodness, you already love the city. You have nothing to lose; trust me on that. And hey: maybe there you'd find someone who can appreciate you. Speaking of which--
Ah, nevermind, I'm running out of time. There's so much for me to tell you! I know you'd understand, and even if you don't trust people, I know you'd trust me. Spend your time with people who appreciate you; don't give in to this "tolerate" bullshit just because it's what seems only available: you're better than this, and it's about time you start treating yourself like you are. If you can't find them in the uni, go ahead and try new places, offline or online. Start that journal everyone's talking about: verbalizing helps. Keep doing what you love even if you don't feel like it: that's the only way to move forward. Take classes. Learn stuff on your own. Do what it takes to make progress happen. Fear will only let you this far.
There are so many things to tell you, but for now - apply yourself, and apply to the NSU. That's the best you can do for yourself. I'm sure you'll figure it out. And make sure to save some money every month. I bought my very own laptop that way.
* ЕГЭ - Единый государственный экзамен, or Unified National Exam. Students take ЕГЭ after graduating from school.
* МТС - a Russia-wide telecommunications provider, one of the three major ones.
* "football" being the worldwide meaning of the world.
* Аура - "Aura", one of the biggest malls in Novosibirsk.