This is called stage phobia.
If continue practice then inshallah you will overcome.
What you do stand in front of mirror & speak any lesson, speech daily for 5 minutes.
The 2nd thing whenever you go on the stage, remind yourself that you are better from all who are sitting & listening to you. If it is not then you will be sitting & somebody else will be speaking.
Always read bismillah before you start. Inshallah you will be alright.