The best way is practice -- you simply get used to anything the more you do it. When I was in HS, I joined debate club, where we spoke for 5-10 minutes at a time in front of anywhere from 5-20 club friends or opponents. When you do this repeatedly, for most people, your anxiety and nerves eventually go away, and it's just "what you do" now. The same was true of musical performance: I hated performing until I did it about 50 times. Then I got comfortable and my average performance was no big deal. Another idea: which language are you native in, and which are you using to speak confidently? If it's not your primary, that could be part of it. There's also focus-related disorders, if you have ADHD or similar, then your symptoms would make sense.