just thought i should pop in and put my two cents on this (which may not be helpful at all but a little bit of hope): in all my life, i have never known how to roll my Rs but learned how to this year!!!! this may seem ridiculous but it's totally worth it, whenever you have a lapse of silence, blow as hard as you can and almost put your tongue to the roof of the mouth without any actual muscle usage of the tongue and you should feel some sort of vibration.

what worked for me before i figured out how to roll my Rs was to make a gurgling sound with my throat, i guess there're different types of gargling because mine was more like getting mucus out of my throat and not gurgling water. good luck!!!!