I can relate... I used to be able to say "thank you!" so easily, but in the past year or so it's become one of my feared phrases. I've taken to saying "hank you!" and hoping they don't notice I'm missing the "th" sound. I know avoidance of sounds isn't what my childhood speech therapist would want, but I just can't handle the "th-th-th-th" noise while the person I'm thanking just looks blankly on... and I end of biting the sides of my tongue from trying to force the sound out.