Go to junior college, or a community college for your first 2 years. You will learn many skills you will need to run your mechanic business, and you could even get some real-life training by working part time for a local shop.
Then, when you have your AA, you will be ready to run a business, BUT if time has changed your mind about wanting to be a mechanic, then you will already have your first 2 years of college finished, and you can do your last 2 years at the university.
I think this would be a win-win solution. Talk it over with your folks. And always remember, they aren't really trying to run your life for you...they just love you and want what's best for you.