I am in my third year of undergrad and my GPA is slowly going down. I have a couple more finals until this semester ends. I got a C in Phyics II and a C in Biology II last semester and I told myself no more!! But, I just finished my Orgo final and didn't do well at all!! I just found out I got another C!! I studied so much for this final and I thought I got it all down!! Before this semester I had a 3.45cgpa and 3.4sgpa, and it's most likely going to go down a bit. I'm planning on taking the DAT sometime in the spring/summer of next year and applying. Should I take it again? I'm already registered for Orgo2 next semester. I'm so worried right now and my hopes of getting into dental school are diminishing.

I'm in pretty much the same boat as you - junior year, 2 C's in prerequisite classes, will probably be getting another C in a science class this semester, but the GPA's still right around average for dental school matriculants. Do you feel like you understand orgo? Would retaking it help you get a better grade in Orgo II? Will your school even let you repeat it if you got a C the first time around? That's really all that matters. When I was in this situation (trying to decide whether or not to drop and repeat Orgo 2 before Biochem) I met with professors who taught the next class and asked them whether or not they thought someone like me would be prepared for their subject.
If you feel that you understand it then your DAT will tell whether or not you know the subject. Don't worry about proving it by retaking the class; prove it by doing well on the DAT.
As far as the downward trend, I personally am taking a
slightly easier courseload next semester so I can make straight A's and have time to study for the DAT. A good, solid semester is really all you can do at this point to show you have your act together. If you don't have time to study adequately for the DAT then so be it; your upward trend (from the middle of junior year to the end of senior year) will be there to support you if you have to reapply. And really, would taking a year off be such a bad thing?