How can I stop thinking so much?

I had just graduated from high school last week, and as of now I don't know what to do with my life. I'm going to college, but nowadays I just feel nervous and afraid about being on my own. I'm not mature at all whatsoever, either. I'm 17, never had a job, nor my license, never aggressive and does everything at the last minute. It doesn't help that I'm insecure and don't like myself very much. I turn 18 in two weeks, and it only increases my fear of being on my own, seeing I'll be a hopeless adult, nevertheless an idiot. Sometimes I just wish to lie down and die in my sleep or just don't feel like living anymore, due to my fear and nervousness. I don't know what to do to solve my problem, and it's been bothering me since graduation. What can I do?