Is it normal that i can't talk to people?

I can't only respond to people in either short, stunted sentences or long-winded sentences that go in circles and end up sounding like gibberish. It's like the words get stuck in my throat and only parts of them will come out. My psychologist told me to basically practice what I'm trying to say in my mind before actually speaking. This doesn't help much because I always end up concentrating on it for too long and there's just this big open gap of silence in the conversation. They usually think I'm ignoring them, and sometimes I do ignore them because I get caught up in thinking how they'll react (even if it's a very casual conversation) and I just decide that it's better to just not say anything at all. I'm not much better with talking online (to people I know; I'm fine with strangers) or texting. I really can't talk to anybody out of a professional environment (I can talk to my psychologist). I'm starting marching band in two weeks, and I am absolutely terrified. I have no idea how to ask someone for a ride when I don't have one, and it's too late to back out. Please help?