High grades are irrelevant - that just means you can regurgitate information in a form the teacher wants.
Now down to it. There are two ways you can own your own business - successfully or unsuccessfully. Your second paragraph is the way to unsuccessfully own your own business (I would give it 6 months). To run a successful business, you have to start with an idea hat can be translated into making a profit. For example, if you are the only auto repair company in town, odds are that you will make a profit...if you are one of ten auto repair companies in a small town, you will struggle unless you can be better than all the others. Once you have identified an idea you think will make money, you then have to identify how it will make money - are you competing on price, on scarcity or product on customer service. Next you need money - one of the main reasons companies go under is lack of cash flow. Next is absentee ownership and finally, lack of experience. Sorry to say but until you identify your idea and then get experience in the field working for someone else, you will probably be among the 4 our of 5 new businesses that fail to make it 5 years. So, basically, you put your work for yourself dreams on hold until you get experience working and taking orders from someone else.
Now, assuming you get this far (idea, competitive edge, experience), you get to start the hard part. Starting your own business means cutting all expenses to the bone (remember, you will have no income for a good 6 months while you build your business) and working 60 - 80 hours a week, by yourself building you business, getting clients, getting suppliers, negotiating with clients and suppliers over every last thing. You get to do this 8 hours a day from 9 to 5 and then, when you stop, you get to work on the books, plan for the next day, etc. - there's another 2 - 3 hours a day plus weekends. so forget free time. And even if you do all this, there is no guarantee for success (hence the 80% who fail).