I experience this a lot when I'm intimidated or like you, around exam time. Sometimes I get annoyed at my own self for this because I think of what to say and then it comes out like garbage, it's so embarrassing. Like they say, some things sound better in your head than when you actually say it. When ever I have to write an essay for an exam, I usually make small jot notes and then start off like that, it usually helps a lot.