Yeah man, I had a lot of trouble sharing in a meeting and was horrified at the prospect of speaking at length. Couldn't string 2 thoughts together, everything I thought of saying would fly out of my head leaving me with a big nothing and everyone in the room looking at me. But it slowly got better, I made a determination to always speak when my turn or if asked and by repetition I slowly lost my fear.
What helped a bunch for chairing a meeting was recommendations as in previous postings on writing things out and practicing, but what really helped was marking the prepared text where I should take a breath and then adjusting the marks as I read it over and over for practice. Then, when giving the speech I know where the pauses are and have confidence that I know what I'm going to say and how. Part of my panic was not knowing where to take a breath so I'd rush and get freaked out. I've chaired about 6 or so times now in a few different meetings and the abject paralyzing terror has faded to general discomfort which is a huge improvement.
I generally offer a silent prayer before I start speaking, that I speak my truth and stay out of the way... its helped a lot.