Hello,
Thank you for asking your query at DoctorSpring.
I assume that you are a healthy individual who is taking a normal Diet. So it is highly unlikely that you will have a nutritional deficiency (including Magnesium).
You might not like what I say here ! But it seems you might be having a Nongeneralized or circumscribed variant of Social Anxiety Disorder. This has nothing to do with your overall confidence. Many individuals including public speakers and politicians. This will require specific Cognitive Behavioural Therapy with or without medications. Beta blockers might not help.
I suggest getting an expert help.
Hope this helps
Please feel free to ask followups
Thank you