feeling nervous, pretentious and lost in public

Hello!

I'm not sure this is the only section that this problem would fit into, but I think this is the most appropriate. The problem is, I feel a huge and unmanagalbe gap between how I feel and think in private and in public. Each time when I have to be in public (and this often does not concern only public speaking, but also socializing in general) I feel that a huge wall separates me from the situation and the people in it. I feel like a small and helpless child who does not understand what is going on and how to act. It seems that I see myself as if from the outside and as not fitting in the situation. I often cannot feel the situation and other people, I feel completely lost in my private thoughts and emotions. It is as if I had disappeared when other people were there. They overwhelm me. As a result, I can't communicate, I feel as if I were somewhere else or as if in a dream. If I am forced to communicate in such a situation I automatically become pretentious and talk some kind of nonsense, as there is nothing in my head.
I am woried about this first of all because it is hard to make connection to people both in social situations in general and more specifically at the university. Second, this is a huge obstacle to my capability of public speaking. There has actually been a case where I had to be one of the speakers in an event and it turned out very bad. I think I was badly prepared, but I can't even figure out what exactly happened. It seems to me that even when preparing my speech I was so closed that I prepared it 'for the people there outside' and did not hear my own thoughts. As a result I felt as I spoke that the speech was messy and I was acting pretentious. When somebody asked me a simple question in a provoking manner, I could not say anything sensible. I started to talk some nonsense as if the question had been very complicated and looked and felt completely stupid. It was as if I saw and felt from the outside how I looked and acted in this situation but was unable to change anything about it.
Maybe I didn't explain the problem as well as I could have. I am not sure I understand myself what is going on in these situations, but I feel I become very defensive and completely lost. Could somebody please give their opinion of what is going on, why it happens this way and how to fight it? I would like to be natural and confident in social situations, I want to feel organic like one person, so that I can make connection to people and not feel the need to hide away.