I believe you are not the only person in the world who cannot roll her R's. This could be as a result of not being able to roll your tongue. Although the condition is genetically inherited, here is a link that could help you, http://www.wikihow.com/Roll-Your-%22R%22s.
Your question reminds me. The very first word type sound my son learned to make was the rollll that is made in the back of the throat for some Welsh sounds. It sounded hilarious to hear baby-babel with such nicely rolled sounds.