The problem may be a relaxation/anxiety thing--however, you may be one of many, many, singers who never learned to sing the "right way"(meaning in a way that gives you optimal voice control, and at the same time, doesn't strain your voice)--If you're making a living by singing, and have never had any vocal coaching at all, you may want to look into it--which is to say, read up, ask around, etc and consider your options(not"find the first idiot that comes along claiming to be a vocal coach and give them your money and do every damned thing they say")--