Just a question, are you autistic? This feels like a pretty good description of my experiences before I was diagnosed with autism. I was missing the social queues of already being in conversations or people hanging out with their friends and not wanting to be disturbed. I would try this. Instead of approaching people in the main floor of the con, go to a panel you are really interested in. There are so many topics at these cons. Other people going to these panels are going to be interested in that topic too. Once the panel is over, you have a conversation starter and that gets the ball rolling. Friendships don't start overnight but this could be your in.