One of the major contributors to lack of confidence when speaking is what's going on in your head.
That is, the internal picture you're seeing, or story you're telling yourself, as you prepare to speak and then attempt to speak.
Imagine for a moment the following landscape (or another that you personally don't like):
Imagine you're there and notice what you notice about how you feel?
What's happening in your mind? What about your body?
Now consider speaking as you hold this image in your mind.
How confident do you feel? How easy is it to consider success, achievement and connection to the audience? How easy is to imagine what you want to say.
For many this picture represents the little voice inside that's saying "you can't do it" "you're not important" "no-one will listen" as your energy plummets and your fight of flight mechanism kicks in, and shuts off access to the part of your mind that knew what to say.
Now imagine this landscape (or one of a favourite landscape)
What difference do you notice? How do you feel? What about your body?
As you imagine speaking with this image in your mind what happens to your confidence? What happens to your expectation of success? Connection with others?
This image represents the internal cheerleader, the voice that says "go for it" "you can do it" "woooooop" and allows your energy to soar and words to appear effortlessly to convey concisely, and inspirationally, your message.
The only thing we changed between the two experiences was the picture. I suspect many who have problems with speaking have an image similar to the first in their mind. The key therefore is ensuring the picture in your mind is one that lifts and inspires you, and the negative chatter can't help but be silenced.
Next time you're in need of more confidence - consider the landscape that you're imagining to represent the situation and change it for a more resourceful one.
Please note: You may want to trial a number of different landscapes ahead of time until you find the one that's just right for you. It might also be context specific - with a conversation with your boss needing a different landscape to one for when you're standing up and speaking to a room full of people.
Do let me know how you get on.