[musictlk] question about singing in latin

[Kelsey Nicolay]

Hello,
Our choir is singing the Lord Nelson mass in Latin this spring.  
I've sung in Latin before and I know that the r is either flipped 
or rolled.  Our director is very strict about this.  
Unfortunately, I can't do either.  I've tried putting my tongue 
at the roof of the mouth like you're supposed to, but it doesn't 
work.  I can't get a sound out.  I can't flip my r either.  The 
only thing I can do is use the letter d as a substitute.  People 
have tried to explain to me how to flip the r, but I still can't 
do it.  Does anyone have any tips that have helped you with 
figuring out how to do this?
Thank you,
Kelsey Nicolay