How can I become less self-conscious at work?

Recently I've noticed that I'm extremely self-conscious, to the point where I am doubting myself before speaking at meetings or team events. In private, however, I am very confident in my abilities, intellect, value, etc, and it shows in my performance reviews. But for some reason, that confidence isn't translating when working with others. In fact, it seems to be making it worse. I am highly confident when working alone and terribly lacking in confidence when doing anything with more than a few people. Anyone else struggle with this or have any ideas how I can become more relaxed in group settings?