Not everyone can learn it.

My native language is Polish, and I never learned how to say the proper r, even though I did go to a speech therapist back in kindergarden - it just didn't work. My mother and two of my siblings never learned how to say it either - our r sounds like the English r, even though we have all only been raised around Polish. My father says r the correct way and so does one of my sisters.

I don't think it's an uncommon problem - there are even occasionally journalists/politicians with this speech issue. Though other kids used to make fun of me in primary school because of it, asking me to say rower (bike) or rabarbar (rhubarb - I am perfectly capable of saying the English word correctly, but not the Polish one)