my co-volunteers are driving me nuts. I feel not listened to .... I feel worthless, useless - all those things made to feel constantly as a child.
Sounds like a massive EF to me. Re-feeling what you felt as a child is a good clue there. That doesn't mean that your co-volunteers are in the right or anything. I'm not supporting them. If you have to keep reminding them of something they ought to know (devolved nations and legal systems), that will get super-annoying.
I want to help those without a voice (I'm a lawyer) so don't want to walk away but those close to me say that it is making me ill. I don't want to be a quitter but I feel like a failure because I feel like I'm not getting through.
Could those people close to you be right? If you get really ill, you might have to walk away anyway (been there, had to do that) and then it would take you that much longer to be able to start volunteering again. (My experience anyway.) Would it be possible for you to step down from this volunteer job for a while? There's that idea of helping somebody else only when you're strong enough, often illustrated with the example of a parent in an airplane putting their oxygen mask on before they help children with theirs.
You are not a failure because you are not getting through to some people. It could well be based on
their comprehension problems. Also some people just don't
want to understand, or they could be narcs. They could be gaslighting
https://outofthefog.website/top-100-trait-blog/2015/11/4/gaslighting or any number of other emotionally abusive behaviour. e.g they could be scapegoating you.