Hi! I don't want to alarm you, but this is you from the future. No, I'm not here to warn you about nuclear war or anything like that, so just keep your shorts on. I know you've just dropped out of school and church, and you're scared and wondering if you've made the right decision. This is probably the last point at which you'd want to get a letter from the future, because you think I'm going to nag you about the importance of a good education and the importance of church and how I desperately regret dropping out of both.
If that's what you're thinking, you are absolutely wrong.
Leaving that oppressive hellhole is the best thing you will ever do. In fact, you're going to be tempted, in a few months, to go back, but don't do it. Complete waste of time - you'll leave again after like a month, because the same old problems will come back. Stay home and focus on your friends and your family. You will, as you already suspect, learn more from your computer than you ever did in school. And you'll have absolutely no trouble going to college - if you truly want to. If you need to mourn for the prom, go ahead - but know that most people's memories of prom aren't actually all that special, and you're probably not missing much. It's just a carrot held out to keep you running toward the end of high school. You will have plenty of opportunities for dressing up and dancing throughout your life.
As for friends: You know those girls you hang out with at school, the ones who didn't give a shit when you came back after being out sick for a month? I want you to listen very closely to what I am about to say: Fuck. Them. You heard me. Fuck 'em. You don't need them, they never liked you for who you really are, just as you've always suspected. Cry if you need to, mourn for all the time you wasted sucking up to them and for the comforting illusion of friendship that has now been shattered. But don't look back. You're not losing much.
Now that I've just about broken your heart, let me put it back together. Those people you're chatting with online, wasting time dicking around on a Pokemon forum? They LOVE YOU. Or at least, they will love you. It sounds strange now, and it goes against everything you've ever been told about having a social life, but I promise you these are the people you need to hang onto. That gut feeling that these are "your people" is not a mistake. A certain few of them may be your friends forever - but they will ALL impact your life in ways you cannot now imagine, and it will all be for the better. And you will, as you grow up and are able to go and explore the world, find even *more* friends. More than you will know what to do with, even, and they'll all love you *because* of how weird and crazy you are, not in spite of it.
Listen to your body. This mono thing is going to come back. You're going to have all kinds of bullshit with your ovaries, too, and your blood sugar. Just relax and try to take care of yourself. Hating yourself for not being as productive as other people isn't going to make you more productive; it will only take away what little energy you have left for doing good in the world. Be gentle with yourself.
Also, let me go ahead and save you years of confusion: Liking long-haired pretty boys doesn't make you a lesbian. They're boys. You do like girls, too, though. You're pretty queer. Be proud of that, even if it isn't safe to tell anyone in meatspace yet. Tell your online friends. They won't mind one bit. And get your ass to the queer youth center as soon as you can drive. You'll be glad you did.
On a related note, getting out of that church was a good move, too. Have fun playing around and learning about different religions, but don't take them too seriously, and don't let them convince you of anything your heart knows not to be true. If a religion is making you *less* of a good person, something is wrong.
Wear whatever the fuck you want. Do you actually like those too-tight Wet Seal prissy clothes, or are they just what you think will make boys like you? Pro tip: There are no clothes that will make boys like you, at least not the kind of boys that are worth it. Any boyfriend or girlfriend (or plain ol' friend) worth their salt will love you in band t-shirts and baggy jeans and ridiculous, Boy George-esque makeup, and black nail polish, and whatever silly thing you felt like doing to your hair this week. (This also goes for whatever music, TV, etc. you like. Be yourself.) Keep playing dressup - the friend who teasingly called you a "freak" for doing so is going to be your best friend in the world in a few years, and she doesn't actually care what you do in your spare time.
Some more about dating: Don't be afraid to get your heart broken, but when it inevitably happens, remember it's not the end of the world. That person you like right now may seem amazing to you, but they are not the only person you will ever have a chance with. As for sex, it really is as great as you think it's going to be, but only if you're doing it with someone you trust and feel comfortable with. Otherwise it's just an awkward mess that makes you feel embarrassed the next day. And yes, condoms matter as much as people say they do.
You are absolutely 100% right in your attitude about work and school: They are NOT all there is to life. You might want to write that fact down, carry it in your pocket, tattoo it on your arm so you can't forget even when everyone is telling you otherwise. Life is about trees and friendship and weird foreign food and strange music and bright colors and incense and thunderstorms. "When you grow up, your heart dies" is only true if you allow it to be. Do. What. You. Want. It's not at all selfish to fill your own happy-tank in order to have joy to share with others. If you're sitting in the dark, you cannot cast light on anyone else. As corny as it sounds, your heart is going to lead you to great things. Follow it.
Having given you a (hopefully encouraging) glimpse into the future, let me ask one small favor of you in return. Go give out some hugs. Hug Mama, hug Nannie, hug Aunt Evelyn, hug everyone in your life even if they piss you off sometimes. You'll miss them one day. I'll spare you from the mental torment of knowing when, but it will be sooner than you ever expected. Visiting a nursing home is depressing, but not as much as knowing you'll never see someone again.
And, last of all? Never, ever stop watching cartoons just because you think you've gotten too old.
P.S. - Yeah, you do grow up to be someone who uses phrases like "worth their salt" and "meatspace." Sorry about that. If it makes you feel any better, your vocabulary also includes a remarkable number of variations on the word "dick", including some most people haven't heard of before. You'll have some fun with that.