Seems you suffer from anxiety disorder. It can be treated..

There is very popular method to treat it.. Let me summerize it as follows-

1. Think that this fear as a third party hypothetical creature like a scary animal, imagine it.. and give it a name - lets say 'foo'. This means you are normal person, but this 'foo' attacks you and make you feel like that (increased heart beat, sweating, trembling, fear) .. whenever you are about to talk in group.

2. Medically it has been established fact that no one dies because of social anxiety, stress. This means, foo can not kill you no matter what worse it may be... it just scares you, makes you feel chill in your body.

3. Third step is to make a list of all symptoms you experience when foo attacks you, i.e. when you feel nervous while speaking in group (dry mouth, chilling weaves in legs, increase heart rate, sweating... ).

4. Next and Important step - foo will not hold grip you forever. It will vanish soon .. possibly in few minutes.

OK, with this preparation, now you are ready to fight the war..

1. Next time, when you have to talk in groups, think in advance that - foo will attack you and get mentally prepared for that fact. As expected, foo will show up.. Just accept foo, don't resist.. let foo do what it can..

2. When you talk in group, foo will definitely show up. Wait for foo to appear. Let him come .. Feel him..experience each symptoms.. since you already know what foo will do, it is not surprise or new thing.... and say.. oh. yeah.. I know that too... Tell him (internally), "oh.. mr. foo, so you are here again ? I knew you will be.. And I also know everything, what you will do to me.. It's fine. No worry.. do whatever you can. I am ready to handle." Flow with it.. don't resist.. foo will try its best to scare you this time too, but since you already know its all symptoms.. you just neglect them.. foo will gets tired himself soon. Don't pay attention to it. Instead ASK FOR MORE.. "Hey, what else you can do ? that's it ? common...."

GO ONE STEP AHEAD ..

Don't stop there. Instead of waiting foo to show up,  be warier .. start searching for foo yourself. Go one step ahead, and do anything you were not able to do just because foo used to scare you (you will not die doing that..).  Hunt for foo yourself now.. Could not go on the top of tower and look downwards ? Could not ride roller coaster ? Now is the chance.. Do it now.. . Let foo show up again. .let it come.. feel the symptoms of foo (which you already know..)..experience it...enjoy it.. Do everything that makes foo appear ... Say "hey, where are you  mr. foo? so are not you comming again ? you can not scare me anymore.. .. "

Practice this.. expect foo.. ejoy his drama.. his effort to scare you.. soon foo will not be intersted in you..

For best effect, imagine foo as a funny charater. Like a duck .. quacking around.. and trying to scare you..

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It sounds funny, but it is effective method.. try it!