Sounds like you are suffering with some degree of social anxiety. You are hyper-aware of others around you, and have convinced yourself that whomever you are talking to is being judgemental of what you have to say....or of YOU in general. I promise you this isn't the case. We are always going to be our own worst critics.
Even in the event of a full blown panic attack (for us PA sufferers) where we think everyone around us thinks we're nuts...most people aren't even aware that it is happening. that's how "unaware" most people are of others...it's reality.
I think some talk therapy and some assertiveness classes will fix this right up for you. You are young and feeling out the world for the first time as an adult, really. It is a scary and unsure time. You just need to grow some confidence and learn to realize that you are an interesting person with interesting things to say to other people. You have to learn how not to focus so much on worrying about what others are thinking. There will always be those who judge you (even though most won't)...you just need to learn how to tell yourself that those people don't matter...and you need to surround yourself with supportive people who share your interests.
Well first you need to realize EVERYONE IS DIFFERENT!! I am a person who is known as a "Stranger to noone" because I can talk to anyone and usually do! I have met many people like you (so you are NOT unique) I say this because, you really need to know that It is OK to just be a listener! without people like you I would not get to hear myself so much!! Please don't think I am taking your situation lightly, I am not, I just want you to know, ALL YOU HAVE TO DO IN LIFE IS BE YOURSELF, if this is you, so be it! there are many different people out there. I see from your other post you have met a girl (we do not need to get into that, that is not for this forum) So that tells me you have to have some social skills, I have met guys and girls who not only can't talk to people, but can't even have a relationship, no less a one nighter. My point is , lighten up on your self, I am not really even sure if your question is for this forum? Do you have Panic Attacks or continued Anxiety that interfers with your life?? are you on meds? seeing A Dr. medical or Psych?
At 20 try going to different places where people meet, open restuarants, bars, (this is just to observe and see the many different people and how they interact with each other, you may even want to go to any self help group, just to listen, see how people open up.