I thought I was gonna die before my 18th birthday. When I turned 18, I felt absolutely no different.
Your issue is not whether or not you feel ready, it's based on your standards for an adult that you don't feel ready for.
Let me run you through my day. I get up, take a shower, hang out with my fiancee, go to work, come home, get on my computer, go to sleep, and do it all over again the next day. Sure, I have to take care of myself, get dressed, plan my future, and all that jazz... but so what? I mean, is that it?
Adults don't have so much responsibility. Taking care of kids isn't as hard as it sounds either, because after you do it a for a few weeks, it becomes so second nature. Not only that, but don't put so much responsibility on your shoulders at once. You may be turning 18, and you might have to start filling out paperwork yourself, but you live and you learn. You don't have to immediately move out, get a job, get married, have kids, grow old and die. You have YEARS before you have to start worrying about everything, and even then... take each day in stride, because it's really not that hard.
This year, focus on getting out of school and getting a job. If you have a place to stay, just do that. You should know how to count by now, add up everything you have to pay, work more than that so you can save up and still have some YOU money, and live your life. Plan for the future, but live for today. It's pie

.