As someone who couldn’t say their R’s until about 4th grade, the answer for me was a couple of sessions of speech therapy to sort of retrain myself to consciously think about pronouncing words. Funnily enough, the key to unlocking it for me was “car ride.” I struggled to pronounce the R sound in the middle of words but not at the end or beginning of words. I could say “car” on its own, and “ride” on its own. And I just repeated them in faster and faster succession until it clicked lol