Overcoming social anxiety is difficult. I tried some volunteering last year. I'm actually from a similar part of UK as you so it could have even been the same organization (I won't name them here as don't want to breach any confidentiality). I signed up as wanted some practical experience in the mental health field for possible post grad courses. (The irony is that I have my own MH issues so volunteering to support others is a strange idea really).

The social anxiety made it unbearable. The post was over subscribed with volunteers and the initial training sessions took place with sooo many people in the room. Sounds silly but something as simple as not having anywhere to sit or panicking over whether I'm taking up someone else's seat, making accidental eye contact/not enough eye contact, all on top of interacting with completely new people made me feel overwhelmed with anxiety. Then I start thinking my anxiety is making people look at me weirdly which makes me even more anxious and the familiar cycle takes hold.

Also, similar to what you wrote above, sometimes when I'm around really confident people (and there were several of those there) I find it frustrating that my SA makes speaking out if I disagree with them or putting my point of view across really difficult. I know that if I force myself to speak, the emotion makes what I'm saying sound too intense so I just stay silent and come across as passive and compliant when I'm actually really opinionated sometimes, especially about political stuff.