Speaking and tight throat

I used to never be nervous talking to people, I was like the class clown but when I got to 9th grade I began to get dizzy in class during speeches or talking to people and my chest would turn red. I am 22 now and when I talk to people my throat gets tight and it makes me sometimes stutter or not have a strong voice. I think that is why people say they think I am 16 years old. I hate this! I want to be confident when I talk to people and have people think I am the age that I am. I hate the tight feeling in my throat also. What can I do?