Ask yourself what benefit disclosing your social anxiety on your application gives you as an applicant. Social anxiety is actually fairly common so I don’t think it gives you any kind of a out on extracurriculars. It’s unfortunate that you didn’t get treatment earlier but it’s great you’re getting it now, I would just move forward and try to make your past activities look as good as possible. I don’t think there’s any shame in disclosing social anxiety but I don’t think it’ll do you much good. My daughter has learning disability and she disclosed that in her essay mostly because it’s very much a part of who she is and where she’s been. We knew that was risky but also felt felt that schools who would discount her because of her disability were schools she didn’t want to attend anyway. How do you feel about that? Would you take an increased chance of rejection in exchange for a better fit? Do you feel like acceptance of your social anxiety is part of what’s needed in a good fit School?