Not to be the "Debbie-Downer," but it also may be an excellent opportunity in your educational career to decide if dentistry really is your ideal career. You don't need to prove that to me, but I highly suggest seeking out other possible careers. See if those interest you, if you could see yourself doing that as your career and be happy with your life. If you find something else, GREAT, pursue it!
However, if you come back around to dentistry, you will realize that A- in any class won't cut it anymore and the only way you will reach it is getting LOTS of A's. This *should* be enough motivation for you to study hard enough to get A's and nothing less for the rest of your undergrad education and hopefully, that will be sufficient to pull your GPA to a reasonable level for application to dental school.
This is exactly the process I went through. I happened to come back to dentistry. I almost didn't. I'm still not 100% (I'm 98%) sure it was the right choice, but I'm in it for the long run. Don't shut out your other options just because you have told yourself and everyone else that you are going to be a dentist, you have plenty of time to still change your career goals.