Cavity questions.. Really scared..?

So when I was little I had alot of cavities from drinking to much apple juice. 2 years ago when I was 14 I had 1 cavity. I'm freaking out because I may have another. Im so worried to lose my tooth or need a root c. The cavity I can see and it doest start hurting intil I bite a few times. Should I be worried? Calling the dentist tomorrow by the way. Also I get dark spots on my teeth all the time but my dentist says there not cavities and didn't show up on the X-ray can they turn into cavities? Thanks :(