I have a similar situation , it's like I have to rehearse my phrases before I walk up to a grocery store/food joint so that I don't mess up my order. What really sucks is when -as you mentioned in the post- I freeze , I feel like a moron with a gaping face , just staring at the shopkeeper , and then after a pause of 2-3 seconds I can finally muster the courage to say something. No matter how hard I try , I always have to add fillers like 'uhh' , 'umm' before I say stuff , like a signal to the brain to get ready.