The problem is that what you have described as Systematic Desensitizationmay not work. It may not be a matter of "mind over body", but rather the body is suffering from a metabolic disorder that can cause the mind the be the victim of but an unhealthy body not producing feel good neurotransmitters.
Having stage fright, as when a person has to give a speech, may be a phobic reaction which is a subclass of anxiety attacks. This may be caused by the body over-producing adrenaline as a result of hypoglycemia.
This can be overcome by going on a hypoglycemic diet. One immediate remedy, although not a permanent cure, would be taking glycerine prior to having to give your speech.
Thee treatment for anxiety attacks caused by hypoglycemia is explained at: