I.L.
Mommy it is going to be okay!
I have been exactly where you are at and then some! My daughter was three and it was several cavities. I felt like such a failure! I did all the things your supposed to; brushed 2x/day, no juice, no soda, very limited sweets and she still had cavities. Devastating.
I had a great pediatric denstist and that made all the difference in the world. They were pros at easing the child and the devastated parents. One thing that they explained to me is that part of dental health is in our genes and the bacteria in our mouth. Some people are just much more prone to cavities than others.
The more I thought about it the more that made sense. My brother, growing up, might have brushed his teeth once a day and never flossed. He had his first cavity at age 34. I, on the other hand, brushed at least 2x/day, flossed regularly (I have floss in my desk, my house, my purse and my car--can't stand things between my teeth) and my whole life everytime I go to the dentist there is something new. My habits are good, it's not my fault.
Just the same, do the best you can and let it be. Be happy that she is five, and it's only one cavity. Think of how much worse it would have been if you didn't take such good care of her teeth!