Confidence and anxiety don't have to be related.
You can be extremely confident, and scared as hell on the inside.
In fact, that's often the case....
Think about that enough, and you'll figure it out.
Separate your external impression, from your internal feelings. Talk yourself up, and when you get nervous and anxious, use that rush to make yourself stronger, but push it back a notch, so as to not come across as a total jerk.
You could also find people that are extremely confident, and ask them how they do it. You'll often find that they're very willing to tell you. Not only that, but they think it's a big secret, and will tend to be discreet about your talks.
I would suggest you get into some therapy to help you get over what MAY be a type of social phobia.
CBT (Cognitive Bahavioral Therapy) I feel would be most beneficial for you.
Peace
Greenlydia