I couldn't roll my the "R" until age 20, after two years of studying Russian, so don't feel bad!

There was an exercise suggested by one of our grad-student Teaching Assistants that did help me: the tongue-twister Eddy, Teddy, Freddy!, said over and over.

The idea is that the repeated "D" and "T" sounds put your tongue into a position that's similar to a trilled "R," so your tongue gets confused and rolls the "R" by accident.

And it did work for me, although it took a couple months, as I recall! But whenever I had some privacy, such as in the shower or walking by myself between classes or jogging for exercise, I made a habit of chanting Eddy, Teddy, Freddy! Eddy Teddy Freddy! Eddyteddyfreddy!, faster and faster.

And eventually, after a lifetime of NOT being able to trill the "R", one day I just did it by accident:
Eddyteddyfr-r-r-r-r-reddy! And with a little more practice I was able to do it perfectly, whenever I wanted.

P.S. After that, the only remaining difficulty for me was remembering NOT to trill the soft "R"! To work on the soft-R / hard-R difference, there's a famous Russian скороговорка (tongue-twister) about a guy named Грека who puts his руку (hand) into a реку (river) in order to grab a рака (crayfish, crawdad).
But first, practice your rolled hard-R with Eddy Teddy Freddy.

P.P.S. If you get bored with Eddy Teddy Freddy, you can invent your own tongue-twisters using lots of "D" and "R" sounds. For example:

Dodo Frodo, Dreidel d'Oro!

...which, as Hermione Granger could tell you, is a magical spell for turning a low-IQ Hobbit into a golden Hannukah-top!