Four drills to help when preparing for a speech
There are many things that can be done to help if you have difficulty giving presentations. I have worked with many people with this exact same problem (with very different symptoms). One individual, for example, could not even say 'hello' when someone came and introduced themselves, let alone speak in front of more than 3 people... Another example is here and here.
Prepare your speech in a specific way. write out the speech, test it and give it a full dress rehearsal. Then immediately do another dress rehearsal, but this time with no sound ... Make all the gestures, mouth and face movements etc, just don't speak the words.
2. Tricky questions
Once done, imagine you get asked some questions - start with simple ones like "what's your name?" and lead up to very in depth, personal questions such as "What colour is your underwear?" When you answer these imagined questions - focus not on your answers, but on your physical state. You know what question you most want to avoid or you have the most difficulty with - have these on your list!
Get comfortable with silence. When someone asks you a question, and you pause to gather your thoughts (or in this case, get comfortable) you will be considered as giving a better answer.
4. More preparation
Independent of your speech, practice changing your emotional state. Pick two emotions, happy and sad, for example. Imagine you are happy, stand and move and act as if you are. Do that for 2 minutes. Then change to 'sad'. Move, talk and think as if you are sad. Do that for 2 more minutes. Then go back to happy for two minutes. (finish with the enjoyable one...) Really amp each state up, even if you don't feel it. How will a world class actor do it?
These are just a few drills from a long list that will give you the skill of intentionally being able to keep control over and change your state quickly and easily. At the start it might be slow and difficult, the more you do it, the better you will get...