Imagine never feeling comfortable, ever, except with alone (or drunk, or (sometimes) with family). Social anxiety is kind of like having a one of those nausea-inducing crushes, except it holds for the whole human race: you want to make a good impression, you're hyper-aware of everything wrong about you, and failure is your worst nightmare but you can't stop thinking about failure even though it makes failure more likely. So of course 90% of your day is spend imagining and remembering past and possible failures.
Sometimes you manage to do or say something cool, but then you have to immediately run away so you don't fuck it up right after. Sometimes you decide to cut your losses and not even try. That's always easier.
Almost every action is on the conscious incompetence level of the four stages of competence. You know you suck, you know you suck, you know you suck. Or worse, sometimes the stages get broken and you believe you suck and you feel that you suck but you don't actually know if you do. All you have is a constant sucky feeling without adequate proof.
You do know that everything, even smiling, takes extra brainpower. People can probably tell that you have to force yourself, and some will dislike you for being "fake."
The part of your brain that says yes, I am secure and comfortable and loved by others never quite switches on. If you are clever enough, you can reason your way into believing that you're safe, into believing that people want you around. Sometimes. Like, only if they actually return texts and stuff like that, start conversations with you and check in when you withdraw.
And you're going to make social mistakes because they're necessary to learning, but mistakes during adulthood mean missed job opportunities or lost wages, falling behind in life because of the ways you've already suffered. The solution is often exposure therapy, but sometimes you feel guilty for testing things on people:
"Will this person mind if they're the first person I've ever asked on a date?"
"Can I tell them that their behavior makes my anxiety worse?"
"Should I thank this person for helping me overcome my fear by actually showing up?"
"Should I hide that this took all my courage?"
And since social anxiety isn't agoraphobia nor general anxiety nor stage fright, you may be perfectly fine in the anonymity of crowds, but break into a cold sweat when you receive a phone call. Or you may be in your element with public speaking, presentations, and other professional situations that can rely on scripts, but have no idea how to relax or "hang out." Or maybe acting is freeing in a way you rarely feel in the day to day, because it is a mask. But then you go home and can't leave your room and have to skip dinner because you can hear your landlady through the door and you can't deal with another person being in the kitchen the same time as you, especially because you feel like living garbage in the vulnerable state of sweats or pajamas.
I personally wonder if people want me around when I'm not being "useful." A couple friends indicate that they want me around for my own sake, but for most, people there is an unspoken obligation to perform, give, or act in specific ways. Even if the obligation isn't there, I always feel it. Maybe it is always there.
Nothing completely erases the suspicion that most people are only friends with me out of pity. Nothing can turn off the hypervigilance that makes my eyes, my mind constantly scan for signs of rejection. The suspicion does grows quieter, very nearly vanishes with people I can read well. It also alerts me to people I should distance myself from, so that we don't hurt each other. That's one of the upsides of anxiety, that is it a deluxe version of the Gift of Fear.
Something like that.