Blocking on the R sound

Blocking on the R sound It started in highschool when I was asked some question by the teacher and my reply started with a word that starts with R but for some reason I couldn't get it out and started making funny noises and everyone laughed. Prior to that I had no trouble with that sound. Now when I want to say a sentence and it starts with R and I'm concsious that it starts with this sound, I exhale heavily and can't get it out. However I have no trouble with R if it's in the middle of the sentence, despite it being the first sound of the word. What can I do to help it?