My muscles also tend to cramp and I run out of breath very quickly. In practices I usually have no problem scoring, but when game time comes I become almost useless.
Every person has what's called a different activation curve.
In Sports Psychology this is called the Inverted U Hypothesis. Basically it says that everyone can be stressed to a point where they perform poorly, whether it be sports or in other mental tasks in life. The best you can do is to work on things that will help reduce your stress. In most cases this comes because you are focusing on the wrong things and not enjoying playing. All of your energy and concentration are going into fear of failing.