Hi @BlueBay 
For what it's worth, I can relate to a lot of what you talk about.
Mental health definitely does have an impact on our ability to understand stuff, I think. As my mental health continues to decline, I find it harder and harder to understand stuff. I'm finding that I need to sit and re-read things several times, and still I find that I don't understand what's being said.
I'm pretty good with regular conversations with actual people. But official documents/articles/statements? I definitely find it much harder to get anything from them today then I did 5, 10, ro 20 years ago. I think that, as the brain gets more and more tired of life, everything becomes more and more of an effort, including concentrating on stuff. That's my theory, at least.
Also, what frustrates me is that vocabulary seems to be changing every five minutes these days to accomodate all and sundry. I used to be able to call therapists "shrinks" (the common term of my generation) - until I offended somebody by saying that. So now I need to call them "therapists"; and I've even been pulled up by people telling me that I'm using that word incorrectly.
And now we are hearing phrases like "person-centered treatment"... What on earth does that mean? Was the old way of doing things "parakeet-centered treatment"? Were all us mentally ill/ suicidal/struggling people being referred to vets by mistake? 
Jokes aside, we're now using "person-centered" because nobody can agree on a term that doesn't offend all and sundry. Some people don't like the term "patient", some don't like the term "consumer", some don't like the term "client", and a lot of people don't like the term "mentally ill". So now we are stuck with confusing language that offends nobody and tells us nothing.
This isn't just confined to the realm of mental health, but exists right across our society at the moment.
Also, I can relate quite well to your experiances of "not knowing all the things that a grown adult is just supposed to know".
I have a lot of problems with geography, and other stuff. People expect you to go to places that you're just supposed to know the location of - then act like it's all your fault when you don't go there; because, as a grown-up, you're just "supposed to know" where to go.
Same with other areas of life. Your just "supposed to know" how to fix things, "because your a grown-up", but I've just never understood.
So your not alone, @BlueBay 