What you say isn't far fetched at all. Social anxiety is such a lonely feeling. You can be in a crowd of 100 people and still feel like you stick out and nobody likes you.
I don't see why you couldn't try a different AD and a Vitamin D tablet. They aren't mutually exclusive. BUT I would be careful with the Vitamin D. Since it is a fat-soluble vitamin, it's going to remain in your system for a while. If you have a healthy diet (fish tuna, fortified milk) and are out in the sun regularly (20 minutes a day), then you are probably getting enough. If you are taking a mega dose everyday, toxicity can damage your liver so I would be careful.

I would say 99.9% of my stress comes from not knowing what career I want and the side effect of worrying about it nearly drives me crazy. So I am in the same boat with you. But you are not doomed. One day at a time has to be your mind set. Otherwise, living in the future (which you can't see), will rule your existence.