social anxiety

I don’t really know why I have such bad social anxiety. literally the only thing I can do when talking to someone is say one word. because if I try and keep the convo going I start stuttering, mumbling, shaking, etc. and I feel like I always make something awkward. like my older brothers bsf who stayed with us a little because he was visiting said goodbye to me and literally the only thing I did was say “bye” and awkwardly wave like ugh I could’ve atleast said “bye, it was nice to see you” or something like that. and literally the littlest things like this always make me overthink it the rest of the day. I couldn’t even make eye contact with him or anything.